How big is this pump? I get only 4.3% drop for a 240v 15A load (no transformers) using #4, and that's a huge pump.
What's the mains voltage at the house? 240V?
I'm with electrofelon, forget the step up and step down transformers.
You'll get a more efficient system using the #4 THWN cu fed from the panel in the house and use a boost transformer at the pond location to feed the pump motor. I would make sure the buck/boost transformer uses cu windings, not AL.
Size EGC accordingly per 250.122 (B) Table 250.122.
Drive a ground rod and bond it to the EGC
The buck/boost transformer is wired as an auto-transformer so it may not matter if it is wired on the line side or the load side of the GFCI protection for the pump motor. May not?.... I would install the transformer on the line side of the pump motor GFCI protection though.
NEC 682