you can add a low profile sata PCIe card to have more than 4 sata devices.

Optiplex XE2 SFF 315w psu is c/w 5040 (and 7040, 9020 SFF).

It's okay if you're building with existing components you already have.  But, starting from scratch with an SFF chassis is not a good idea.  Beside PSU and SATA controller, there is no drive cage nor cooling to setup 4 or more hard drives.  The 5040 only supports two 2.5" drives and one optical.  If using SSD, each drive will cost more than the 5040 itself.

I suggest to look into a cheap Precision tower.  Example, a 5820 normally has powerful PSU (950w) and plenty of space for 3.5" drives (at least 5).  By default, it has 6 SATA ports for HDD and 2 SATA ports for ODD.  Best of all, the 5820 has front-accessible, hot-swappable flexbay for unlimited access to your hard drive library.

power math

Operating: 3.5-inch NAS drives typically consume 6 W - 10 W each during normal operation.

Spin-up Spike: At startup, each drive's motor can momentarily draw up to 15 W - 20 W (especially the 7200 RPM models).

Total Peak: Four drives peaking together can demand up to 80 W. Add a low-power CPU/Motherboard, and your absolute peak is 150 W.  

More drives can be connected via USB 3.0 using external enclosures, whether the enclosure is for a single drive or multiple drives.