The upgrade is worth it if any of the updates are useful to your wokflow. Best thing to do is to download the trail, then you have 30 days to see if you like it.
We've release updates to our current extension to be CS4 compatible, so for WebAssist extension you should be fine.
Extension Manager CS4 offers you the ability to be install your existing extension installed in previous versions. At least it does for CS3, not sure about from earlier versions. You would need to serialize the WebAssist extensions again, as that has to be done for each install, even if migrated over from a previous version. You can get your serial nubmers from your order history page: orderhistory.php