When I release a custom control which works in both Silverlight 3 and Silverlight 4 is there any reason to provide a separate binary version targeted at Silverlight 4?
As far as I understand SL4 apps using SL3 controls run normally in SL4 mode but maybe there are some hidden benefits of recompiling controls for SL4 (or gotchas of not doing so).
No need to rebuild: the "quirks mode" actually is based on the version that the application's built against, not its components.
Be warned that if there is a breaking change in the XAML that would affect your Silverlight 3 control when it moves to Silverlight 4, even if you don't rebuild the control, if it's used in a v4.0 app it could throw an exception at runtime.