Hi Michael,
I don’t think it has anything to do with your RPi4. I have a RPi3+ and it also works only on one IP setting at a time.
My understanding is that you essentially asking to communicate the same thing twice but through two different channels and the system simply doesn’t know which one has priority over the other. Which means it just clashes with each other. There are some who claim to have it working, but I suspect you would then have to set one as TELEM A and another to TELEM B. - Just my thought without trying. But that’s where it is getting tricky because only the A type has the fast 115000 bit rating. So obviously that would have to be the one you want to use for sending control signals as it is important to reach drone fast.
My other thought is once you receive your USB telemetry kit the problem may be also sorted as this is essentially seen as a serial connection, rather then a IP connection. - But be aware that this also only usually runs on 56000 bit speed.
Anyway, just to make things more interesting: If you ever consider using a live camera (Rpi camera or USB camera) you will have even more challenges ahead of you as the mentioned transmissions speeds are not suitable for the required speeds. Such system will only work over a good WiFi connection or 4G / 5G Mobile phone network connection.
…thought I will mention it just in case you’re tempted with that.