I work with Pharo 4 and no longer use Pharo 3, its very stable I have not experienced any kind of problem with it except one with autocompletion when I was hacking Pharo themes for my Nirreas project.
Definetly recommend it to you to switch just for GTSpotter alone and many other new nice things.
There million ways to help, you can a) bug fix c) document d) make your own tools and share them with the rest of us e) introduce pharo to other people so we can grow as a community and the list goes on and on.