I need to continue creating Silverlight 3 content, but also want to create content for Silverlight 4, is it possible to have Expression Blend 3 and Expression Blend 4 installed at the same time?
Yes. Expression Blend 3 and Expression Blend 4 install and work seamlessly side by side. You may also want to test the multi-targeting capability of Expression Blend 4 which enables you to target either Silverlight 3 or Silverlight 4 as your target platform while still taking advantage of many of Expression Blend 4’s enhanced features.