3 and 4 will always be the leanest because of the way the manifold is set up, how ever should not be that noticable, as Nev said it will be a mixture thing, make sure to calibrate the carbs with a balancer, if it is only in a certain range, it could be a slightly blocked jet, ect. do you have egt's on all pipes, if no.3 if hotter it is definately telling you it is lean.
You might need to go right through everything, make sure you check the balance tube between the maniflods for the slightest of cracks, it could be just enough to really lean out no.3.
check and i mean really check your carb sockets(replace if in doubt), i had a 912 run like a dog shot in the a** at idle but over that beautiful, turned out to be a crack on the bottom of the carb socket(black rubber that holds the carbs to maniflods) at idle it was bouncing and leaning, at revs it was sucking enough to hold the crack closed, it is very possible it is doing the opposite here.
Have fun dont you just love it when its something really crappy like this :)