Really I think a lot of the differences amount to bells and whistles. The 35 series is their premium line of compact utility tractors. It is a little more powerful, a little heavier, has a bit more reach and lift capacity, larger tires, bit more hydraulic flow. I'm not 100% sure but I think the 4010 comes with hard lower links on the 3 point, I'm quite sure the 4035 has the telescoping (is what I have and love them). I think the open station 4010 also has a non-foldable ROPS which the 4035 is foldable (didn't matter to me, but it you have a low door to pass through it matters).
Working on hills, having larger tires and having them loaded with fluid lowers the center of gravity of the entire machine and assists stability greatly.