If you’re out of SATA ports or M2s you can buy PCI adapters.
If you’re buying SSDs they’re small and don’t care about orientation, can but plugged into the cables and stuffed anywhere in the case that doesn’t impede airflow.
Where do you want your drives? What sort of drives? I’ve also found it more performant to stuff them in the case and 4 drives isn’t a stretch unless you’re also running a ton in the target server.