there are some observations here on FCE 3.5 and 4.0:
Also check out the apple page on FCE 4 for info on the new features:
I know one thing you can do with FCP 6 is batch capture that can't be done with FCE 4.
It might be worth spending some time comparing the features from these two pages:
For starters, FCP 6 is part of FCP studio, so you get Sound Track Pro 2, Motion, Colour and DVD Studio Pro 4 & Compressor 3 (which aren't included with FCE 4).
Sound Track Pro 2, IMHO, is more stable and if you use it a lot is worth the upgrade. However, it's only included as part of FCP Studio or Logic Studio.