AB350 Gaming K4 - Ryzen 9 3900XT Support ?

Honestly, even if it does I would not put a 3900, hell even a 3700 in a b350 board.
The B350 boards were designed and built around the requirements of first gen Ryzen.
Power delivery will be weak and you will most likely experience high temps on your
VRMs and thermal throttling.

This is likely the main reason AMD did not actually want to support 300 series
chipsets with Ryzen 3000. This is just my personal advice mind you but I wouldn't
put more than a 3600 in a B350 board.

For the 3900x/t and higher I would go with an x?70 chipset, ideally an x570 as it
will enable you to take full advantage of the 3000 series architecture (PCIe 4.0).

On the cheaper end I would say a B550, something like the B550 Taichi or the
B550 Steel Legend if you want to go even cheaper. I would not recommend less than
the Steel Legend however. While even the B550 Pro4 should be able to handle the
3900x/t it will run hotter and will likely not be adequate for 4000 series and
higher CPUs of similar spec or higher in the future.

In all honesty however, I would go for the 3600 on your current board and hold out
till DDR5 becomes standard. The 3600 should be more than adequate for most tasks.
The only reason I could see going for the 3900 or higher would be if you are doing
serious CPU encoding heavy tasks.

Check reviews and look at performance in the applications you will be using the
system for.

The key with high core count CPUs is to look at the VRM (power delivery system),
you always want to go overkill rather than under. I tend to avoid the more budget
oriented boards for high end CPUs.