I reported this bug https://connect.microsoft.com/VisualStudio/feedback/details/686863/devpath-is-not-respected-by-net-4-when-running-web-applications#details
I set up a directory containing an assembly, set the DEVPATH variable to point to this directory, and change the machine.config to enable the developmentMode.
If I then run a winforms application that references the assembly, I can see from Debug/Windows/Modules that the CLR has loaded the version in the DEVPATH directory.
If I run a web application inside Visual Studio, then the version loaded is one from inside the ASP.NET temp directory. It appears as if the DEVPATH has been ignored.
If I target .NET 2, then the DEVPATH is respected in both cases, and the DEVPATH directory version of the assembly is loaded. It seems that it is not respected under .NET 4.
This bug is marked as resolved even though I received no notification that you needed more data. If you need more information please just email me to let me know.