Here are some amazing new features for garage band 4.
Multi-Take recording: I can honestly say this is a very useful feature. When I try to lay down new parts I ALWAYS use this feature. I can't express how helpful this feature is.

taken from
http://www.macworld.com/article/59419/2007/08/flgarageband.html
Automation: You can now automate almost everything. You can even change the tempo of a song. A useful feature that has never been in any other earlier versions of GarageBand.

Magic GarageBand. This is the most child like feature of GB4. It just plays 9 preset songs, and you can change the genre, and backing instruments. It may be useful for the non-musician, but that is about it. I guess they added it because most MAC user's don't see a purpose for GarageBand, so they added this novelty feature.

There is a new arrangement tool that allows you to organize a song by segments. For example, the typical verse, chorus naming system. I think this is very useful, but I haven't played around with it much yet.

Keep in mind that GB (garageband) is also marketed as a tool for podcasters. This is a very smart move
for apple, as the number of podcasters is increasing exponentially ever year.
You can also choose between 16-bit or 24-bit audio resolution qualities. This is also a very welcome improvement.
I would like to thank
http://www.macworld.com/article/59419/2007/08/flgarageband.html
for inspiration and enlightenment for this article.
Buy GB software at...
http://moogprodigy.com/Synthesizers/Apple%20iLife