If you're going to put two guys with plasma guns in, just commit to shooting and don't worry about losing a single non-power weapon attack per model. Give the librarian gate so they can leave if they do get assaulted.
If the squad is in a rhino, it may be worth going 2 plasma, 2 melta, and then firing out the top hatch whichever two guys are most appropriate for your target - only hop out to fire all four if you really know you won't be shot/counter-assaulted, and can kill everything in the area you're firing on.