Mechanically, 4600 and 7800 are very similar. I've taken both apart, but some of the materials used, especially in the inner workings, is different, 7800 having the lighter/more exotic materials like carbon and titanium.

I haven't had either for a longer period of time, so can't speak about long-term performance or durability, but it's difficult to tell the difference other than if you held one in each hand you could tell that 7800 is lighter. I am having to repair the 7800 right shifter, so perhaps that speaks to the durability of the unit, at least in my case.