Sounds like it may get crowded when they get to full adult size. I'm not sure what variety you have, but I have seen them get about 6". Might be a chore to keep the water clear and excessive nutrients from building. I think a bigger tank might be better if you can get one.

A power filter is best with goldfish because of the waste issues. Either a hang on box filter or canister filter will work fine. as far as the current being too strong for the fish it depends on what type of goldfish they are. Single tail types like comets or subunkins like good water flow while double tail types like black moors or red cap orandas enjoy more calm water.

At some point those fish will out grow the tank but for right now the best thing to do is to make sure the water quality is good.

With 4 goldfish, yes their going to outgrow the tank. As far as the power head paparoseman is right it depends on the type.