I was looking a few months ago and went for the L4600 ( I was lucky and able to find one). I believe the 4701 is a 4 cylinder and the 3901 is a 3 cylinder engine. The 3301 and the 3901 use the same base tractor, just different engines. The 4600/4701 completely different tractor - larger heavier tracor and components.
I'd recommend the 4701 hands down - you can never have too much power.
Good luck in making your decision - keep us posted what you do
Dan