why 4 GB RAM for the OS?The server should have at least 8GB of RAM and about 4 GB dedicated to Java/FTB. More RAM helps the OS but you don't want to dedicate to much more than 4GB Java/FTB.
You mean the java process uses 16 GB's of RAM?You can never say exactly how much RAM a server is needed. It depends on how many chunks are kept antive how many players, mods, etc etc etc ...
For my little DW20 server with a maximum of 3 players runs purely currently 16 GB RAM if no one is on-line ...
Must you always see yourself. If you think you know exactly wiele chunks will keep active, you skin directly purely more RAM.