Are you looking at an eHydro or geared machine? I have a couple reasons that I would go for the 4520...regardless of price (within reason).
1) The 4520 and 4720 have a different fuel injection system. Take a look at JD parts and you will see the difference. The 4520 and 4720 have a much lower component count.
2) Due to the different fuel injection system, the 4520 and 4720 have a feature called eThrottle. When enabled, it speeds the engine up as you press down on the hydro pedals. I find it very handy for loader work and pulling.
3) More power.