Documentation suggests that ext2 and ext3 filesystems may be converted: https://ext4.wiki.kernel.org/index.php/Ext4_Howto
I have not tried this myself though.
For simply ext4 support in kernel: Kernel 18.104.22.168 on my VL box mounts and reads it just fine. After upgrading e2fsprogs to version 1.41.3 (sw 12.2) I can also create ext4 systems.
As far as your root filesystem.. I agree with M0E-lnx; pick a version that supports ext4 during installation. Converting afterwards would be complicated. I really don't see any good reason to do so.