Q: What is the maximum number of ships one colony may have. Four right?
You can have more than 4 ships on a colony. Imagine if you recover all your ships from the warp due to Mobius Tubes, but you only have one colony -- it would get all 20 ships on it.
Q: What is the maximum number one can attack with. Four right?
Yes, you have a limit of 4 ships on the hyperspace gate.