Whistler,
I have had both, and although I liked the 3720 a lot, the 4520 I now own is better for my needs. I would say the main difference is the ride and stability, although the tractor does seem to have more power also. I have not found that the 4520 is less manuverable than the mid frame machines, but if one was working in really tight spots, maybe. My advice: measure where you have to go and if a 4720 fits, and fits into the budget, I would do it. You will not be disappointed. My understanding is there is a wait, but there are enough machines around to get one on locator. My gut feeling is the 3720 would be harder to find, as the one I had and one other are the only two I have personally seen. Word is they are tough to find. You might be looking at a couple months on that machine.
John M