Has anyone experienced/solved apparent problem with Quicktime 4 and Avid Cinema? I can only get Avid to run with Quicktime 3, yet I encounter other situations that demand Quicktime 4 which necessitates some very awkward switching of preference files in the sustem folder.

Any ideas

Yep, according to Avid, the Avid compression card for PowerMac's is utterly incompatible with QuickTime 4 - they specfically told me that there was no update planned. I ripped the card out of my Mac faster than you can say "Bye bye Avid".

Anyone want to buy an Avid compression card that only works with QT3?

What version of Avid Cinema are you running? If it's the USB or G3 software version, QT4 is supported (The G3 version requires an upgrade).
The Performa version and the B&W/ATI XClaim VR education bundle don't support QuickTime 4.
Avid has a page that breaks it all down. It's here: http://www.avidcinema.com/support/quicktime4.html

Hope that helps,

-Chuck