Check with the guys and see what they have more fun doing but if you had to choose, I would go with 3 and a short fielder either way because if you have a bunch of punch and judy hitters, you have someone to pick those off and if you have power hitters, the three outfielders have a little time to go out and get the ball.
this only works if they do it by accident imo.
if the batters are hitting the ball short on purpose, chances are they can hit to the opposite side of the field as where your rover is standing.
there just isn't a lot of times a 'true' 3 person outfield and a single short fielder make sense. even when we do use a rover(which is actually most of the time), we treat it like the name rover implies. with one batter they back up and become the fourth fielder, with another they stand behind 2nd, sometimes they play right down the line, in coed the girls are allowed to bunt, so sometimes they will play right behind where 3rd would be, and 3rd is pinching in to cover the bunt.
a good 'rover' plays in a different spot on the field for every batter, and in some cases that changes based on the situation as well(2outs with a runner on 1, we play differently than having the tieing run on 3 with 1out...)
then again, I used to play on a team with my parents and their work friends, and well, we'd have somebody play rover, and then get them to stand where we knew the ball would never go so they didn't get hurt...