With the release of Silverlight 5 it is of course now difficult to find a download for the latest Silverlight 4 Developer Runtime for those of us developing for restricted client environments (again).
Silverlight 4 Developer Runtime 4.0.60831.0
Silverlight 4 Runtime 4.0.60831.0
Silverlight 4 SDK 4.0.60310.0, April 2011 Update
http://download.microsoft.com/download/B/9/B/B9BDD218-6974-4816-A309-0FC85A105521/sdk/silverlight_sdk.exe (from here: http://www.microsoft.com/download/en/details.aspx?id=15636). You should install this after you have installed the Silverlight 4 Tools for VS2010.
The above were found by going to Silverlight.net and selecting the “Silverlight 4 (Previous Version)” -> Silverlight 4 Runtime, however you have to use IE. Using Chrome resulted in downloading the latest version 5 runtime instead. Once downloaded I reviewed the final URL that was used to download, then changed Silverlight.exe to Silverlight_Developer.exe to get the developer runtime as well.