Hi!
As for add-ons It all depends on which one we are talking about.
At the moment I'm using some with ff 4.0b8pre. Namely : Download Statusbar 0.9.7.2, NoScript 2.0.3.3, Forecastfox Weather 2.0.2,
Tree Style Tab 0.10.2010091901. They work as expected. However it might require some altering in xpi - install.rdf - just to change em:maxVersion="4.0b8pre" - so instead of just installing it - download it - change maxVersion to what you need and - in most cases it does the trick. If it doesn't - look for update - it won't take long before all important addons will ff4 compatible.