The V4/V5 thing was not at the bottom of the Airbus problem. We use both V4 and V5, and the Citation Mustang was designed using both products. 95% of the Mustang is CATIA V5, while the rest is V4. V5 can read V4 data with no problem (at least in the 3-D area). Our database is set to automatically create a V4 copy of every V5 part that is written, so people working in V4 can access the V5 data, again with very little problem.
While I do not know what really happened, I (like many others) have heard rumors that it involved the Digital Mockup, or lack thereof. We use the DMU on a daily basis. All of our engineers can call up a significant area of the aircraft whenever they want.
While we all long for our old, glory days (I still miss Applicon Bravo 3 at times), I don't have a single V5 user that would willingly move back to V4 (not after the first 3 months, anyway). In fact, some of our existing aircraft programs are still in V4, and they have trouble getting help whenever they need it, because no one wants to go back.
The biggest problems that I see currently are the Data Migration of Drawings from V4 into V5. It's an ugly mess. Even the simplest drawing revision is now taking many hours simply because of the conversion.