It is general requirement that you want to host your ASP.NET 2.0/3.5 website/webapplication onto the hosting server which provide support for .NET Framework 4 but not for .NET Framework 2. At the time of development we normally assume that by default all application should run on .NET Framework 4 because backward compatibility is supported. But sometimes they are not because of new changes into ASP.NET 4.0. Here is the link for your reference: ASP.NET 4 Breaking Changes.
It is general requirement that you want to host your ASP.NET 2.0/3.5 website/webapplication onto the hosting server which provide support for .NET Framework 4 but not for .NET Framework 2. At the time of development we normally assume that by default all application should run on .NET Framework 4 because backward compatibility is supported. But sometimes they are not because of new changes into ASP.NET 4.0. Here is the link for your reference: ASP.NET 4 Breaking Changes.