The same thing bothered me: we have a large system with approximately 50 services / contracts in several bindings with some complex configurations and user bindings / behavior. I moved it all to 4.0b2 and ran without problems. They did a great job of maintaining backward compatibility when things changed. I have not yet seen the list of changes from 3.5 to 4.0 (only between beta versions), but I'm sure that they will publish it in the release.
Good luck