My XP laptop that does not have .Net 4 just updated Razor and still works properly.. so that's odd then. But also my installation was done with our client installer which handles a few razor permissions, none of which really apply to XP but still...
Also, Razor was not recompiled for .Net 4 so there's not even a requirement there. Also if it was a .NET error there would be an error message saying as much, and the program wouldn't run.

But if the fix works then great.