Best 4-Bay NAS: Where the Parity Math Starts Working

Four bays is the tier where the arithmetic finally works in your favour. A two-bay mirror gives you one drive of usable space out of two, which is a 50 percent tax on everything you buy. Four drives in RAID 5 give you three drives of usable space, and four bays is the smallest count that can run double parity at all. That is the real reason this is the volume tier of the whole category, and it matters more than any specification below.

Three picks below, then the capacity arithmetic that should decide your layout, then the enclosure specifications that genuinely differ. Read the mixed-drive section before you commit to a brand, because it is the one place where the choice of vendor changes how many terabytes you end up with.

Best 4-bay NAS by what the array is for

Beginner Cheapest four-bay worth owning

Synology

Synology DS423 (4-Bay Diskless)

$399.99

Four bays, DSM and SHR at the price of many two-bay boxes. The Realtek chip rules out transcoding and heavy container work, which is the trade you are making.

Best for: File storage, phone backups and photos where the array matters more than the processor.

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Intermediate Best four-bay for most builds

Synology

Synology DiskStation DS925+ (4-Bay Diskless)

$743.71

The box most four-bay buyers should own. SHR and SHR-2 both available, ECC memory, and the only scheme where a mixed drive set counts fully.

Best for: An array you intend to grow one larger drive at a time over several years.

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Expert 10GbE and NVMe pools

Asustor

Asustor Lockerstor 4 Gen3 AS6804T (4-Bay, 10GbE)

$1,517.99

10 gigabit networking and M.2 slots that can hold real storage pools rather than cache only. It costs more than four 8 TB drives, which is the number to weigh it against.

Best for: Editors moving large files who already own 10GbE on the client side.

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What does four bays actually buy you?

Parity efficiency is the number of drives of usable capacity you get per drive of parity, and it improves with every bay you add. A two-bay mirror spends half of what you buy on redundancy. A four-bay array under single parity spends a quarter. That is the whole argument, and it is arithmetic rather than opinion.

Computed with the same engine behind the RAID capacity calculator. Usable figures are decimal TB as printed on the drive label, with the binary TiB figure your operating system will show alongside. RAID 10 tolerates one failure guaranteed and two in the lucky case where the losses fall in different mirrors. Rebuild reads is how much data must be read off the surviving members to reconstruct one failed drive.

Note the two rows that read identically. Synology SHR on four identical drives is exactly RAID 5, and SHR-2 is exactly RAID 6. Not approximately, exactly, because with matched drives SHR collapses to a single parity group. Anyone telling you SHR gives more space on a set of four matched drives is wrong, and the entire value of SHR appears in the next section.

When does Synology SHR actually give you more space?

SHR slices the drives into horizontal bands at every distinct drive size and protects each band separately. A band sitting on k drives with p drives of parity yields (k minus p) times the band height of usable space when k is greater than p, and is stranded entirely when it is not. Standard RAID and every ZFS vdev do something much cruder: they truncate every member down to the smallest drive and throw the rest away.

Put four mismatched drives in and the difference stops being academic.

Stranded capacity is space you paid for that the array cannot address. On the same 36 TB of physical drives, SHR returns 12 TB more usable capacity than RAID 5 at the same single-drive fault tolerance.

That is a real difference in drives you have already bought, and it compounds over the life of the array. It is also the answer to the most common four-bay question, which is what to do when one drive is smaller than the rest.

The practical consequence: SHR is worth choosing for the future, not for today. If you buy four identical drives now, every vendor here gives you the same capacity. The difference arrives in two or three years when you replace one drive with a larger one, because SHR is the only consumer scheme that gives you anything at all before the last drive is replaced. Work your own set through the RAID capacity calculator, and see SHR against RAID 5 for the band-by-band walkthrough.

How much capacity does four bays give at each drive size?

Choosing a bay count and choosing a drive size are the same decision made twice, so it is worth seeing them together. The table below prices four drives at every sensible capacity and shows what you end up with under both parity schemes.

Drive prices are the verified listing prices for the IronWolf 4TB, IronWolf 8TB, IronWolf Pro 12TB, IronWolf Pro 16TB, Exos 20TB and WD Red Pro 24TB. Cost per usable TB is against the RAID 5 figure. Prices change often, confirm on Amazon.

Two things fall out of that table. The first is that cost per usable terabyte improves steadily with drive size, which is the argument for buying fewer larger drives rather than filling the bays with small ones. The second is the reason to stop: the larger the drive, the longer the rebuild, and past roughly 8 to 10 TB the RAID 5 column starts asking you to read 30 to 70 TB off three same-age drives to recover one failure.

Which four-bay enclosure specifications genuinely matter?

Most of the specification sheet is noise at this tier. Four things are not.

• SHR support, which only Synology has. It changes how much of a mismatched drive set you can use, and it changes the upgrade path for the whole life of the array.
• Integrated graphics. Intel Quick Sync is what makes hardware video transcoding work. The Core i3-N305 and the Pentium Gold 8505 have it. Realtek chips and AMD Ryzen embedded parts do not, at any core count. See best NAS for Plex.
• ECC memory. Worth paying for on an archive you will not re-read for years, because a silently flipped bit becomes corruption you discover long after the backup rotation has aged out the good copy.
• Network ports. One 7200 rpm drive alone sustains 150 to 260 MB per second against gigabit's 113 MB per second ceiling, so a four-bay array is held back by a 1GbE port. 2.5GbE reaches about 280 MB per second and is the cheap fix. See 2.5GbE against 10GbE.

Specifications come from the published manufacturer sheets held in our NAS model database, not retyped per page. Drive totals use four Seagate IronWolf 8TB at $289.99 and four IronWolf Pro 16TB at $579.99. Memory shows fitted capacity and the manufacturer maximum.

Watch what happens to the spread. The cheapest and dearest boxes here differ by $1,118 on their own, which sounds decisive. Populate all of them with four 16 TB drives and the same two builds differ by $1,118 out of totals well over $2,500, so the chassis choice moves the total by a fraction of what the drive choice does. Choose the enclosure on the features you will actually use, then spend the argument you were having on drives instead.

What about the cheaper end of the four-bay field?

Two units sit below the picks above and both are worth knowing about. The TerraMaster F4-425 at $365.48 is the cheapest current four-bay with an x86 processor, which matters because x86 is what makes containers and hardware transcoding possible at all. The TerraMaster F4-424 Pro at $644.99 carries an eight-core Core i3-N305 and 32 GB of memory, which is specification you cannot buy from the established brands anywhere near that money.

What you give up going in that direction is software maturity and track record rather than hardware. Synology DSM and QNAP QTS have a decade of updates behind them and a large community when something goes wrong. That is a real thing to weigh, and it is also the thing people overweight: the data on a TerraMaster or UGREEN box is on standard Linux software RAID and Btrfs or ext4, so it is recoverable on any Linux machine if the vendor disappears.

The UGREEN NASync DXP4800 Plus deserves a specific mention because it is the value outlier: a five-core Pentium Gold 8505 with Quick Sync, 8 GB of memory expandable to 64, and two 10 gigabit ports at $656.99. If transcoding is on the workload list and SHR is not, it is very hard to argue against.

Who should not buy the expert pick?

The Asustor Lockerstor 4 Gen3 at $1,517.99 is genuinely the most capable box here: an eight-core Core i3-N305, ECC memory, 10 gigabit networking and four M.2 slots that can hold real storage pools rather than cache only. Most people reading this should still not buy it, and the reason is the price of drives.

Run the substitution. The difference between that unit and the Synology DS925+ is about $774, which is two 8 TB drives or most of a 16 TB drive. Capacity is what home arrays run out of, not processing power, and the 10 gigabit port only pays back if the pool can feed it, which four mechanical drives cannot.

The honest exception is video editing off the array, where a 10GbE link plus an NVMe pool is the entire point and the mechanical drives are only the archive tier. If that is not the workload, buy the cheaper box and put the difference in spindles.

What else does a four-bay build need?

Three things get forgotten and all three are cheap relative to what has already been spent. A pure sine wave UPS so an outage during a write is not a rebuild, sized on the UPS runtime calculator. A backup target, because four bays of parity is still copy one and the backup sizing calculator will tell you how much you need. And a 2.5GbE switch if the box has multi-gigabit ports, since otherwise you have paid for a link speed you never see.

Related reading

• Best NAS hard drives for the half of this purchase that actually costs money
• How to choose NAS drives for CMR, workload ratings and cost per terabyte
• Best 8-bay NAS if the library is already outgrowing four bays
• RAID 5 against RAID 6 for the parity decision at this bay count
• How to migrate to a bigger NAS for the drive-by-drive upgrade path
• Usable capacity chart for precomputed figures at every bay count

Deciding on a specific model? We review the Asustor Lockerstor 4 Gen3 AS6804T review, Synology DS925+ review and TerraMaster F4-424 Pro review in full.