Did you want the return pump to be submersible as well? I think there are many more choices for external pumps (often with much better ratings at that height) than submersibles.
If you want a Mag Pump with ~1000GPH @ 4' Headpressure (assuming there are no elbows and tees and whatnot), I would suggest getting at least a Mag 12. You might also want a couple of powerheads within the tank to really stir things up.
Good luck!
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32"x32"x20" Cube-ish tank
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