The simple answer is yes ... you do need all four versions.
In fact ... I am running a 64-bit operating system ... and I have both .NET 3.5 and .NET 4.0 installed ... so I am running exactly what you brought up as an example.
Some additional info: you should use the 32-bit version of Snoop to run applications that are running as 32-bit (and similarly for 64-bit). It doesn't depend on the operating system. That is ... some applications will continue to run as 32-bit on 64-bit
operating systems. It all depends on how the application is configured to run. If you go to your task manager (on the processes tab), the image name is appended with *32 for all applications running as 32-bit.
As to the .NET 3.5 and .NET 4.0 versions of Snoop ... I have noticed that
usually you have to use the .NET 4.0 version of Snoop for applications built on top of the .NET 4.0 version of the framework.
Hope that helps!