It involves building other delicate things like dbus, e2fsprogs, util-linux-ng, etc
See here http://vlcore.vectorlinux.com/trac/vlcore/wiki/FileSystem/Ext4
When we were implementing this in SOHO 6, I recall b0rking my installation a couple of times, but you may have better luck
Basically, your kernel has to support it, and then
e2fsprogs has to be rebuilt so you can perform the conversion/formatting
dbus has to be rebuilt (I dont know why)
util-linux-ng has to be rebuilt for you to be able to mount the filesystem.