Run ASP.NET 2.0 and 3.5 applications on the same server
Yes, you can run both together, infact the 3.5 applications use the 2.0 compiler. Why? Because version 3.5 is just a wrapper around 2.0 with some additional assemblies.
But the newer 4.0 version is a completely new version, and will not be compatible with older versions. That is you cannot compile 3.5 or 2.0 code using 4.0 compiler, IIS will treat them differently too.