I opened the wrong version of my project in Xcode 5 and now Xcode 4.6 crashes when trying to open that project. I only changed one xib file in Xcode 5 and using Xcode 5 hav
It's probably the case that opening the project in Xcode 5 caused it to upgrade the project to a new format automatically (even if you only manually changed one or two files). That upgrade is usually not-reversible. You can probably restore from a backup or from version control.
If you don't have a backup or version control, you should perhaps consider setting that sort of thing up before using beta / developer preview software on production projects that matter in the future.
In that case, you can probably re-create the Xcode 4 project and re-add all your files, et cetera.