If you were using the .NET 3.5 version of mojoPortal you should restore the Web.config file from the installation package and then restore any customizations to that web.config such as machine key. This problem is dicussed in items 9 and 10 on our FAQ.
If you opened the pre-compiled release files in VS and it offered to update to .NET 4, probably the only thing it modified was web.config since the rest of the code is already compiled under .NET 3.5.
Hope that helps,
Joe