I personally prefer MM3 since I feel that MM4 is a bandaid for an improper amount of weight on the rear wheels. While I don't subscribe to the heavier is always better mentality, you do need some weight on the tires to help them grip. A mid motor car has less weight on the rear wheels than a rear motor car does. The solution was to run a 4 gear setup so that more weight is shifted rearwards during acceleration. You can tell this is happening by the fact that MM4 cars can wheelie on nearly any surface. Once you pull the front wheels off the ground, all of your vehicle weight is on the rear wheels. In this regards it works.
Unfortunately many people have a problem with rear end side bite during off power corner entry. The rear end likes to slide out. On high grip this isn't a problem but it can be anywhere else. There just isn't as much weight on the rear wheels and a rear motor car will go right around you here. The solution is to get weight back on them. With an MM4 car you could obviously just get back on the throttle but then you wouldn't be steering . That's a problem in a corner! This huge weight shift taking weight off of the front wheels, the turning wheels, is also why MM4 cars may have a harder time getting back on power out of a corner. A proper cornering technique is to do all of your braking in a straight line before you get to the corner. You should be getting back on power right as you start turning. You can't drive an MM4 car this way. You need to coast around most of the corner unless it's very high grip.
An MM3 car with a little weight added to the back drives very neutral. With no rear kick and that weight, forward grip during acceleration isn't a problem. You also don't have the issue of on power lack of steering in a corner. Artificial weight shift is a bandaid to a greater problem. Unfortunately is causes more of it's own.