I’ve tried several times to boot the DRTLS_raspbian_R2.0.img to a Raspberry Pi 4 and I get the following error at startup with sd card inserted:
recovery4.elf not found (6)
recovery.elf not found (6)
start4.elf not found (6)
start.elf: is not compatible.
As far as I know the provided image is for raspberry pi 3b. From RPi 3 to RPi 4 there have been updates with regard to the boot process. The raspberry pi 4 therefore needs a different image format (larger boot partition, updated bootloader etc.) therefore the images are not compatible.
In theory there should be a way to update the image: