That is a nice fish, funny, I can't help but think it might be a black arowana. They are pretty much identical after they loose the distinctive infancy colouring, but are more slender, slightly more gracile fins, maybe a little more irridescent colouring to the scale edges, just a thought, if you only got it at 4 inches, I think you may have missed the initial colouring. Good bet the store didn't realize it either.