Just an FYI:
With the pi4 you can have multiple kernels on your system at the same time but it takes some time to set it up and much easier to do as @Strit says and install a kernel you want to test from the repo.
You can though download the different kernel packages and extract the kernel and it's module tree:
Put the kernel in /boot with a different name and put the module tree under /usr/lib/modules/. Rem out this line in /boot/config.txt (It is not needed anyway):
# initramfs initramfs-linux.img followkernel
Then add theses lines to /boot/config.txt conforming to what you named the new kernels you added to /boot. Then un-comment only which kernel you want to boot then reboot. An example:
As you can see; a lot of trouble to set up but some people do it.