Side-by-side specifications of the Synology RS3621xs+ and RS4021xs+, from official datasheets. Differences are highlighted.
| RS3621xs+ | RS4021xs+ | |
| CPU | Intel Xeon D-1541 | Intel Xeon D-1541 |
| Cores / threads | 8 cores / 16 threads | 8 cores / 16 threads |
| RAM (base) | 8 GB DDR4 ECC UDIMM | 16 GB DDR4 ECC UDIMM |
| RAM (max) | 64 GB | 64 GB |
| Drive bays | 12 | 16 |
| Expandable to | 36 | 40 (with 2× RX1217/RX1217RP) |
| M.2 slots | 0 (via PCIe adapter cards) | Optional via PCIe adapter cards |
| Network | 4×1GbE RJ-45, 2×10GbE RJ-45 | 4× 1GbE RJ-45; 2× 10GbE RJ-45 |
| 10GbE | yes | yes |
| Max raw capacity | 1 PB single volume | 1 PB single volume |
| Hot-swap | yes | yes |
| Form factor | 2U rackmount | 3U rackmount |
| Released | 2021 | 2021 |
| Power consumption | — | 145.79 W / 62.68 W |
| DSM at launch | — | — |