On the PA-28's I've flown (Warrior and -180) full fuel was 50USG and tabs was 34USG. Handily enough, four average adults leaves you room for 34USG or so of fuel - i.e. as a rule of thumb fuel to tabs is OK unless one or more of your adults is on the heavy side. I guess that's why the tabs are where they are (remember this would have been calculated in the 60's when people were perhaps lighter!).
Of course you should still do a W&B calculation the first few times you travel 3- or 4-up - but you may well find that the above works for your aircraft too.
Tim