That's seriously the only thing stopping you from using Ubuntu? All the other differences doesn't matter, the four finger swipe is the make or break?

There are other things here and there that I don't like about Ubuntu but the four finger swipe is one of those polished aspects of OSX that you don't realize how integral it is to trackpad usage until you don't have it. I would guess with a high degree of confidence that you have never used a Mac with four finger swipe for more then a token amount of time based on your response.

I don't use Exposé that much, just some widgets. It has it's own button.