I don't know, v 4.0 is not even finalized, so trying with v 3.x makes sense, if you are running Windows or have a Linux wihth WINE installed.
At least in theory, using MAC address should be completely independent from IP settings and from the thingy been set as bridge or router, a MAC is more or less "carved in stone" in the device settings.
On "normal" devices (with more than one ethernet port) the device default configuration is with ether1 set as WAN, and all the other ports (including the wifi radios) assembled into a bridge. This bridge (it depends on the specific device and ROS version) may have a setting of "auto" and in this case the bridge has the same address of the first ether port in the bridge, usually ether2 but the common advice is to set the MAC as fixed, and this is also what the initial configuration script does on most devices, but still taking the ether2 address.
Check if your bridge has a statically configured MAC and if it is not set is as so, see Rule #6: