Ext4 has been declared as NON Experimental in the latest kernel, 2.6.28, so it can be used on most system now, as long as you have used the latest kernel. The detailed information about Ext4 can be seen at KernelNewbies. You can see also how to convert your ext3 to ext4. It's safe and you won't lose your data. It's also possible to mount existing Ext3 partition using Ext4 but without changing it permanently. Everything is listed there.
Please note that Ext4 is still in early release and so far, no distribution has used it as the default file system. Probably they will start doing so next year when they have done sufficient testing.