You need to check the app pool of your web application having correct .net framework or not.
Go to IIS and click on Application Pools. Can you see application pool starting with ASP.NET 4.0? if it is there, you have successfully registered .net 4.0 on IIS.
if you don't have 4.0 app pool try below steps in the answer
How to add ASP.NET 4.0 as Application Pool on IIS 7, Windows 7