Developers, artists and user interface designers from all over the world worked together to make GIMP more powerful and easier to use than ever. The changes from version 2.2 are too numerous to list here, please check the Release Notes to get an overview.
If you want to compile GIMP 2.4 yourself, you can grab the tarball from ftp.gimp.org or one of its mirrors. Binary packages for the various supported platforms are becoming available as we speak. Our Downloads section should help you to locate the package for your system.