Are you sure? .net Full isn't available via Windows Update. Only the client profile. If you've installed .net 4 full after SIMS, you might want to run simsapplicationsetup.exe again. I've got an app somewhere that gives you a quick easy prompt if .net 4 full is installed, no requirements - like .net 2. I'll PM you a linky.
I've only enabled .net 4 in WSUS, not installed anything on workstations, and they seem to have upgraded themselves fine. Except for one that turned out to have not been installing updates for months, 101 when I fixed it with proxycfg -d
Managed to fix this yesterday - was literally a numpty moment!
I ran SIMSINST again to re-install/repair it, and that seemed to fix the problem I was having and re-installed the framework for me and the user was able to log in.
I also installed .net 4 full on a seperate PC that was also having the same problems, and they were able to log in after as well.