You've said electric potential twice. So I think you meant to say electric potential energy in the first sentence. In this case, yes, you've understood it.
This is the correct way to do the problem. But your method to get Ufinal was not right. (Although you did get the right answer anyway).
You've got the right idea, but the calculation has a mistake. The plates are separated by 4 cm, so you should convert that into SI units before you use it in the equation, or it'll get complicated to keep track of the units.
You're given the electric field, so you need to relate that to the potential energy. The equation V = kQ / d is true for an electric field created by a point charge, but it is not true in this case, where they are telling us that the electric field is constant throughout space. In other words, you have a much simpler integration to do!