● TWRP Installation
Having it's bootloader locked, the only workaround (AFAIK) to install a custom ROM on the Droid 4, is to install SafeStrap (which comes with TWRP). which is fortunately as easy as was rooting your device.
  1. From your device (as it is the simplest way), download the latest version (3.75 with support for the unused partitions) of SafeStrap from Mentor37's Web Site here or from AndroidFileHost here
  2. With ES File Explorer, navigate to the directory/folder where you have downloaded (probably in the download directory/folder of your SD card) SafeStrap, open it and install it.
  3. Run it and click on Install Recovery, (granting/allowing SuperUser/root rights) and power off your device.
  4. To boot into recovery from when your device is powered down you have to:
    - press the volume up and volume down buttons at the same time (I usually hold the device in my left hand and on those buttons with my 2 middle fingers)
    - press the power button for 2 seconds, which brings you into the boot selection menu.
    - press the volume down button until you get to BP Tools
    - press the power button and the volume up button.
  5. Make a backup of your Stock ROM, making sure that you enable compression.
  6. Start installing a custom ROM :)