I can confirm that Mailcow works perfectly fine on a VPS with 4GB of RAM when ClamAV, Solr and SoGo are disabled…
root@mail:~# free -h
total used free shared buff/cache available
Mem: 3.8Gi 1.7Gi 647Mi 13Mi 1.8Gi 2.1Gi
Swap: 0B 0B 0B
As you can see I have still more than 2GB available on my instance, so you might even get away with less than 4GB.