iPhone 4 doesn't charge, but loud speaker works?

The iPhone 4 dock and speaker assembly are 2 completely different parts. Before the 4, it was one unit but they separated them starting with the 4's. It could be 1 of 3 things that I can think of.

1) The replacement dock is defective. It's happened to me a number of times where a new replacement part is defective which is frustrating as all heck.

2) The connector isn't connected to the board completely. The dock connector has always felt weird to me when plugging it into the board.

3) There is a short on the board somewhere. If this is the case, get a new phone. Buying a new logic board is ridiculously expensive.

Just go through everything that you did and recheck all the connectors. It might help to get a magnifying glass and really inspect the connectors and the inside of the dock to see if everything looks as it should.

Good luck!