There is no ASP.NET 4.0 Support established for the domain in ISAPI mode
while enabling ASP.net 4 support on IIS 6-based hosting.
ASP.net 4 is not fully supported by IIS 6.
ASP.net 4 is fully supported on IIS 7 and 7.5 as mention on KB
Refer to the following knowledge base to learn more about supported versions of ASP.net: