As per docs a minimum of 6 GB is recommended. If you disable solr, and probably clamav as an antivirus scanner too, it should work with 4 GB RAM too. Disabling SoGo would even get more free memory, but you’ll lose any webmail at this point.
There are config options to disable each individual service:
SKIP_CLAMD=y
SKIP_SOLR=y
SKIP_SOGO=y