What ROM are you using?
What recovery image are you using?
Do you have a linux install on any machine?
Assuming you do not have a Linux install already I would get a bootable Linux CD (Jaunty bootable would be fine), put the micro SD in a card reader, then convert from there.
the short version is
tune2fs -O extents,uninit_bg,dir_index /dev/<partition>
I believe cyanogen's recovery image has the capability as well. For that matter his ROM has the disk tools, only problem there is the fact that you need to have the partition unmounted during the process. Therefore my opinion is if you are really set on updating to EXT4, then get a bootable Linux CD.
Yes Cyanogen's recovery image has e2fsprogs, so you could do it from there http://forum.xda-developers.com/showthread.php?t=523558
I still stick with my original recommendation of a Linux install and a card reader.