I got these instructions from page 21/22 on RC groups, for more info visit this thread.
First rewire your Motor 4 Signal wire to the PPM RX as per diagram below
Now upgrade your Betaflight to 3.1.6 or higher so you can have DSHOT in the options under config. Currently Babyhawk is on 3.0.1 which does not support DSHOT.
BEWARE - You will need to reset the quad up from scratch, ADVANCED users only.
Now you need to do some CLI commands to move motor 4 signal wire in the software to the PPM.
Goto the CLI TAB
Type the following
resource PPM 1 NONE
resource MOTOR 4 A15