We may have been able to help you resolve the initial problem you had with .NET 4, you could have done what it says on the basic troubleshooting page to try and find out what error was happening and then we may have been able to help given the error detail.
If your host does support .NET 4, my recommendation would be to use .NET 4 and the .NET 4 version of mojoPortal and see if we can help you resolve the original problem. Eventually we will stop supporting .NET 3.5 (not any time soon but eventually). In any case I recommend .NET 4 over .NET 3.5 to anyone who can get .NET 4 hosting.
Best,
Joe