Hi,
.Net 4 is backward compatible with the previous versions of .net.
It means that is you want to recompile an application that was designed using a previous version of .Net and target the new .Net4 framework it will work.
See here for details:
http://msdn.microsoft.com/en-us/library/ff602939.aspx[
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In another hand .Net2, 3, 3.5 and 4 can happily coexist on the same PC and each application will use the version of .Net it is targeting.
So basically your issue is not an issue at all.
Valery.