Looks like yet another new design. This is what the M4R V3 looks like from the inside and that's clearly different: https://i.imgur.com/73dXHSU.png
What FCC ID does it have on the label on the bottom of the device?
And yes, if you really want to have OpenWrt on your devices then you need to identify the serial connection and solder some wires to that so we can get a bootlog and find out what other device is most closely related to yours.
Back in the day when I tinkered with the M4R V2 I found out that it had the exact same hardware in it as the Archer C6 V2 and thus it was not that hard to figure out how to get OpenWrt to run on the V2. If we find a similar device for your V4 then you can just work on a custom firmware by looking at the code done for that similar device. But for that we need the FCC ID to find photos of what is under that RF shield (or you gently pop off yours. but lets try to find FCC photos first) and then we need a bootlog.