Side-by-side specifications of the Synology RS4017xs+ and RS4021xs+, from official datasheets. Differences are highlighted.
| RS4017xs+ | RS4021xs+ | |
| CPU | Intel Xeon D-1541 | Intel Xeon D-1541 |
| Cores / threads | 8c/16t | 8/16 |
| RAM (base) | 8GB | 16 GB |
| RAM (max) | 64GB | 64 GB |
| Drive bays | 16 | 16 |
| Expandable to | 40 | 40 |
| M.2 slots | 0 | via PCIe adapters |
| Network | 2x 10GbE RJ-45 + 4x 1GbE RJ-45 | 4x 1GbE RJ-45, 2x 10GbE RJ-45 |
| 10GbE | yes | yes |
| Max raw capacity | 400 TB | 1 PB |
| Hot-swap | yes | yes |
| Form factor | 3U | 3U rackmount |
| Released | 2017 | 2025 |