G'day all,

I have an old RPI3 running Jessie that I wanted to replace with a 4
(wanted USB3 & GbE)

Not wanting to re-configure or re-install I downloaded the latest
Raspbian for the Pi4 (Buster) to use for parts.

I replaced the boot partition contents straight from the Buster image.
Had to edit cmdline text to alter the root parameter "root=/dev/mmcblk0p2".

Then I simply rsynced /lib/modules and /lib/firmware across to the root
partition (adding not replacing).

It booted right up with USB, net and console functional and I was back
in business on the new hardware.

I don't use the GPU except for initial bringup, nor wlan or bluetooth.
So I can't vouch for those, although udev certainly detects the wlan.

The config.txt lines to disable wlan & bt still work also :

dtoverlay=pi3-disable-bt
dtoverlay=pi3-disable-wifi

Regards,
Brad