The ext4 file system is now merged into Linux kernel as ext4dev. However, an ext4-formated partition can not be used as a boot partition, because GRUB Legacy(referred to as grub in the following part), which is the most popular bootloader nowadays, doesn't yet support mounting and reading ext4 partitions. And it is said that several popular distros are planning to adopt it in the near future. Some distros already did so, such as Sabayon (an Italian derivant from Gentoo). So it would be worthwhile to enable ext4 file system as grub boot partition.