my way:
1* install backupmenu ,cssu-t , kernel-power , recovery-boot,
2*modify backupmenulauncher.item for ext4(search ext3 and replace with ext4 ,mkfs , fsck , etc, read before replace) , after that copy extrafiles.tar.gz extract it (search ext3 in content and replace with ext4 , MKFS ,FSCK ") after you have edited the files , archive back to extrafiles.tar.gz and replace them
the path is :usr/share/backupmenu
IF THAT IS TO COMPLICATED , YOU CAN USE MY FILES (atached to this post) ,
1) download and extract them to usr/share/backupmenu.
2)reboot
3) slide keyboard and let it open
4)enter to backupmenu and make a backup of both rootfs and optfs
save-it to internal storage "MyDocs" , it's faster
5)reboot
6)go to recovery boot : and type:
mkfs.ext4 /dev/mmcblk0p2 *(wait to complete)
7)reboot
8)enter to backmenu and restore your backup created *(if doesn't succeed from first atempt , repeat steps from 5 to 8)

after a successfull boot , we can tweak the ext4 to it's limits
coming next

Tweaks for ext4
a)install leafpad
open terminal and enter:
root
leafpad /etc/evend.d/rcS-late (search for ext3 and replace with ext4)
reboot
slide keyboard and enter to recovery-console and type:
tune2fs -m 0 /dev/mmcblk0p2 (removes the reserved space)
reboot to maemo
b)
copy mount-opts-overwrite to /etc/default/
copy fstab to /etc/
reboot and voila

files are in the archive atached to this post

A new life for n900 put ext4 plus ext4 tweaks + the other tweaks (hw of hildon , swap on sd , oc, decrease swapiness ,dsp oc plus codecs) and you have a new phone

Is really important to have swap on sd to get ext4 on my phone? Cuz i cant afford sd and i dnt have it. Please tell

OPT as ext4 is it possible?

this is optfs as ext4 , doable on phone without killing yourself with the terminal , when backupmenu restores a backup it formats optfs with ext4 instead of ext3 , that is the magic , sd swap isn't necessary for this
read instructions twice because this mod can make your phone unbootable