You can only load one runtime into VFP. So repeated calls to create an instance of wwDotnetBridge will always use the first runtime loaded regardless of what you pass for a version parameter.
.NET 3.5 is really .NET 2.0 runtime - the runtime version is the same, only new libraries were added in 3.5.
If you start with V4 and you end up with V2 that means that V4 is not installed and the latest available version was loaded (which is 2.0).
Now why you're getting this error I don't know. Are you sure you built this with .NET 3.5 and that it doesn't have dependencies on later assemblies (that shouldn't really be possible). Further are you sure that .NET 3.5 is installed on the target system where you are running this?
I think if you install .NET 4.0 and run under V4 it'll likely work, but this shouldn't be necessary.
Can you send me the assembly and I can take a look at it here...
+++ Rick ---
from Maui, Hawaii