Side-by-side specifications of the Synology RS408 and RS409, from official datasheets. Differences are highlighted.
| RS408 | RS409 | |
| CPU | 800 MHz | 1.2GHz |
| Cores / threads | — | — |
| RAM (base) | 512 MB | 256MB |
| RAM (max) | — | — |
| Drive bays | 4 | 4 |
| Expandable to | — | 8 |
| M.2 slots | 0 | 0 |
| Network | 2x Gigabit | 2x Gigabit |
| 10GbE | no | no |
| Max raw capacity | 4 TB | 3TB internal; 8 bays w/ RX4 |
| Hot-swap | yes | yes |
| Form factor | 1U rackmount | 1U rackmount |
| Released | 2008 | 2009 |
I can’t reliably compare RS408 vs RS409 from the specs you provided, because both spec fields are empty. If you send the CPU, RAM max, bay/expansion, 10GbE, and M.2 details for each, I’ll give you a 2–3 sentence recommendation.