HI,
I have Silverlight version 4.0.50401.0 installed on v10.6.4 with Safari 5.0.1 and so far it hasn't caused any problems.
One thing to add to "rkaufmann87's" advice is to repair permissions after you install the plugin.
Launch Disk Utility. (Applications/Utilities) Select MacintoshHD in the panel on the left, select the FirstAid tab. Click: Repair Disk Permissions. When it's finished from the Menu Bar, Quit Disk Utility and restart your Mac. If you see a long list of "messages" in the permissions window, it's ok. That can be ignored. As long as you see, "Permissions Repair Complete" when it's finished... you're done. Quit Disk Utility and restart your Mac.
Carolyn 🙂