Silverlight plug in missing

[Aspen]

I'm running OSX 10.8 and Safari 6.0.2 with Microsoft Silverlight installed (version 4 since version 5 doesn't work with my other software).

I have a Safari tab open with the software that requires Silverlight, all runs fine. Then switch to a different tab for a bit of browsing, then back to the tab for the software requiring Silverlight but it always gives the message "Plug-in Failure". This requires refreshing the software every time.

Any ideas on a fix?

Thanks much.