You can create an IPA with Rhodes 4.1, but not with Xcode 6 and not for iOS8.
You need at least Rhodes 5.0.30 for XCode6/iOS8/64bit.
Starting with Rhodes 5.0.2 there is SOME shaky support for Xcode6, but not 64-bit until 5.0.30. So, if you need to build for the App Store, you need at least 5.0.30.
If you have to stay behind, you can install Xcode 5.1.1 and build for iOS7. Otherwise, it is not a difficult update from Rhodes 4.1 to Rhodes 5.0.38 (current version).
5 vs 4 does not have big breaking changes. It was mainly a marketing/labeling change associated with the new licensing model. If you updated from 3.5 to 4, it is not nearly the same effort.