Instructions for installing 4ext with custom MTDs:
So:
1. download the 4EXT Recovery Control or 4EXT
Recovery Updater.
2. Choose the version you want to install, but do
not hit "install", only press the download button.
The app will prepare the recovery config and make
sure everything is setup while you download the
recovery.
It will place the recovery into a zip file on your
sdcard.
Copy this file to your computer for later use to
flash the recovery.
3. Then boot into your previous recovery and
create a nandroid!
4. After that, format all partitions except your
sdcard (needed to be formatted are: /cache, /data
and /system)
5. After that is done, boot into bootloader and
unzip the recovery zip file you previously
downloaded and put on your computer.
6. Use fastboot flash recovery or AndroidFlasher
to flash 4EXT Recovery. (please tell me if you
don't have AndroidFlasher et.c.)
7. boot the recovery.
8. Enter the tools menu and select "format all
partitions except sdcard"
9. Reboot recovery
10. Flash your cmtd zip file with your settings
11. Reboot recovery
12. restore the nandroid
13. Finished
These instructions come straight from the dev maddmax.