If your .Net 4 web-apps fail to load, or you do not get .Net 4.0 options for websites under IIS7, then it maybe because your .Net 4.0 installation has been corrupted and it probably just needs to be repaired.
Here is one symptom: Under ISAPI and CGI restrictions for the IIS web-server, Asp.Net v4.0.xxxx does not show up.
Here is a simple command to run the repair:
%windir%\Microsoft.NET\Framework64\v4.0.30319\SetupCache\Client\setup.exe /repair /x86 /x64 /ia64 /parameterfolder Client /norestart
It took about 15 minutes to complete on my machine.
And after the repair, all the enteries showed up: