Yes the files in the /boot partition are for a vast majority of the machines produced that will run that image. If you look in there you will see all of the model .dtb files named after the machine it will boot on, the Pi 3 & 4 included in it. My Pi 4 which runs the pure Debian as an example of what it will boot on.
root@buster-raspi:~# ll /boot/firmware/
total 119688
-rwxr-xr-x 1 root root 23435 Feb 8 15:47 bcm2711-rpi-4-b.dtb
-rwxr-xr-x 1 root root 13990 Feb 8 15:47 bcm2837-rpi-3-a-plus.dtb
-rwxr-xr-x 1 root root 14250 Feb 8 15:47 bcm2837-rpi-3-b.dtb
-rwxr-xr-x 1 root root 14622 Feb 8 15:47 bcm2837-rpi-3-b-plus.dtb
-rwxr-xr-x 1 root root 13624 Feb 8 15:47 bcm2837-rpi-cm3-io3.dtb
Edit: And you should just be able to take your existing card and boot it in Pi 4 because of this universal boot if the Pi 4 files are there.