A 6ft on a 4100 will be no problem at all in any thickness of grass, you will find you will bog the mower itself down before the tractor grunts hard. The 4100 and my 3910 are very much the same tractor and my 3910 doesn't notice it has a 6' mower behind it. In pasture clipping or thinner grass you could easily go with an 8' or even a 10', if steep hills are involved I wouldn't go more than a 7' and that is only because of the weight when you lift it and try to make a turn.