What size wire will I need to feed a 3/4 hp pump?
For a Goulds (CentriPro motor), 2-wire 3/4 HP submersible (5 FLA, 6.2 SFA) 8 AWG copper is recommended for up to 1420 feet (5% drop) by CentriPro. The chart is somewhat vague as to whether that's one way or round trip length; I'm sure you could call and ask, but 6GA is good for 2210, so 4Ga is clearly not required on that variant. Of course, I don't know that this motor is the one you have since you have not revealed your specific pump model.

Three wire motors require an additional wire, of course. Also an above-ground Start/run control-box. They also have higher amperage loads for the same power (6.6 FLA, 7.8 SFA). 8 Ga copper will only get you 1129 feet.

If you have not yet purchased the pump, I'd strongly suggest the 2-wire version.