Inventor 2011 is compatible with Fusion TP4.
However Fusion TP4 is not available anymore on the labs. We have Fusion 2012 available that is working with Inventor 2012.
Probably you can send an email to the labs guys to get a copy of TP4.
Many thanks for bringing out the issue.
Actually, Fusion TP3 and TP4 both can open Inventor 2011 files. It means that TP3 and TP4 are compatible with open Inventor 2011 files. For your issue, can't open all Inventor 2011 files in your TP3? or is it just for specified file?
if just for the specified files, coudl you please send the file to me: firstname.lastname@example.org. Thus i will verify it and give you feedback.
Autodesk ACRD Inventor Fusion QA