The full atx version has some additional pci-e slots and this review says it has an additional m.2 slot
https://www.tweaktown.com/reviews/8759/asrock-fatal1ty-b450-gaming-k4-amd-motherboard-review/index.html
Though I can't see the additional slot so maybe they stack one on top of the other.
The pro 4 (micro atx) and k4 gaming (full atx) it has been said look as though they have the same 3+3 power phases.
They also have usb 3.1 ports.
The gigabyte Auros has 8+3 power phases and usb 3.1 ports so you should easily be able to do 3.8ghz on it with the stock cooler and maybe you'll get to 4ghz with an aftermarket cooler but there's no way to guess if you'd get the full overclock on it.
Then it was said that there's virtually no point in a 3.8ghz overclock because it's the same as the boost speed, so, you are still heading into uncharted waters, from your point of view, because considering the x470 boards have got 16 power phases to maybe help get through to the 3.8-4.2 range, and that if you also get an aftermarket cooler.
If you get a board with more power phases and an aftermarket cooler there is likely less chance that the overclock will fail.
I couldn't tell you if the k4 gaming or pro 4 boards would fail if you tried to push the voltage too high because I haven't bought any or tried to overclock with them but based upon the general information available maybe with only the 3+3 power phases you are immediately pushing the limit if you increase the voltage and if you started to think you could push it some more then maybe you'd find the board fails or at least it shuts down if it has any safety features (wouldn't count on that) but you are saying you don't want hassle so
with the 8+3 phase auros board you have a bit more margin for error maybe a bit more stability because you are at the stage of not knowing where the temps. and voltages will lie it's better to have a bit more of a margin.
The gigabyte auros also has a mATX and full atx version with more pci-e ports and and 2 m.2 ports.
And there are also MSI and ASUS boards to consider. The asus range has had some bios problems according to various threads and the msi tomahawk has been favorably reviewed.
So I'm just saying you seem to have picked the least of them in the pro 4 or k4. and then you're worried about the overclock. So it sounds like there is more chance of problems with your current selection.