Ext4 file system (4th extended file system) is the next generation of journaling file systems, retaining backward compatibility with the previous file system, ext3. Ext4 was released as a functionally complete and stable filesystem in Linux kernel 2.6.28. Ubuntu 9.04 aka Jaunty Jackalope was released with support for the ext4 file system, so do many latest builds of many other Linux distributions.
One of the most evident advantages of ext4 for end users is the drastic reduction in time taken to do an
fsckoperation of your hard drives. The other advantages of ext4 file system over ext3 are - it provides better performance and greater reliability. Also it increases the maximum file system size to a whooping 1 exabyte. Compared to this, ext3 supports a maximum of only 16 TB of file system size.
So it was inevitable that I chose ext4 as the file system when I installed Ubuntu 9.04 Jaunty Jakalope on my machine - dual booting between Windows XP and other Linux distributions. And I had no complaints at all.
Then recently, Microsoft released Windows 7 RC to the public for testing, which was too tempting an offer to pass off. And so I diligently downloaded the Windows 7 OS and installed it on my machine. Of course, as with all things Microsoft, Windows 7 wiped off my GRUB menu on MBR. Repairing the MBR would be a clinch until I realised that to mount the ext4 file system, you need to have a Live distribution which supports ext4. So I was unable to re-install the GRUB menu to the MBR because the Live CD of the Linux distribution I used could not detect the ext4 file system on my hard disk.
Fortunately, I still had a CD with Ubuntu 9.04, which I used to re-install GRUB boot loader on my machine.
Bottom line : A drive with ext4 file system cannot be mounted within older Linux distributions as they don't have support for ext4.
To know more about Ext4, read the following resources ...