Though TPL is widely accepted as a feature of C# 4.0 , but heard that it is not supported on SL4, but SL5.
Could not find any solid evidence of it.
So why the d
If you need TCL in Silverlight 4 you can add the "Microsoft.Bcl" and "Microsoft.Bcl.Async" nuGet packages to your project.
They implement part of what TCL delivers in .net. More info can be found here
Don't forget you have to use the TaskEx class instead.
TaskEx
Hoped it helped.