This is a very common issue when most of us start building robots. The reason why 4wd is hard to work with (assuming your robot is in this configuration) is if all wheels are placed on the corners of the robot you have a problem with turning. The wheels are traveling at different speeds and distances. This is why cars have differentials, so that one side is slowed while in a turn.
Check this website for more info. Our team was able to make 4wd work by placing the 2 wheels on each side almost side by side with less than 6 inch gap in between them. The short distance overcomes the issues I mentioned before. But we have not done this in years and opted to go with 6wd instead.