Is it possible to collaborate on a Xcode 4.x project with Xcode 5, without any compatibility problems between the two versions?
Yes.
There were no changes done to project file so you can safely use it on both versions same time. Xcode 5 might just prompt you for some automatic settings changes - so you should ignore that otherwise Xcode 4 might be asking again to revert those changes.
Source: Grzegorz Krukowski