It depends on what vm you are running and if you are short on memory. You can take a look at your Swap space usage. If it reports no usage then you have not run out of RAM. If you see any routine use, then adding RAM is a good thing and I’d say a must.
With that said, adding a little more RAM allows you more freedom to do a little more if desired.
if you put 4x8gb, they run in dual channel as 2x8gb.
What you shoud avoid is put 4 stick with double density (sorry, prob the wrong name, but i don’t know how usually is in english ), because depending on mainboard they can be rejected.
If your actual kit is 1rx8, with another similar kit you shoud be fine