Just a quick one, seems pretty weird to me.
I installed .net 4.0 on an old IIS6 box, ready to deploy a asp.net mvc app targeting .net 4.0. I thought, which to me seems logical, that I install .net 4.0, setup a new web site, new app pool, set the web site to asp.net 4.0 (other configuration also needed to run MVC on IIS6 here and here) and it would just work.
Errr… No. The page cannot be displayed! Nothing to do with MVC.
Apparently just because you have installed .net 4 and the option is available in IIS, it’s not enabled. I’m not going to repeat anything here…take a look at this post – clear, easy steps on exactly what you need to do and how to check if this is the problem.