This thread from 2018 discussed a couple of options to stream your Pi's picture to another system, but I don't know how much of it is still viable. As you can see there, I tried to do it myself, but I eventually gave up, because it was either not working or too slow on my Pi 3B. Maybe the latter will be better on a Pi 4.
I also tried a external capture card for around 80€, but although it worked, it added somewhat of half a second of delay, which made most games not pleasant to play.
The reason I wanted to stream the picture rather than just use an hdmi splitter was that my 16:9 / 720p video projector doesn't like the native solution of the 4:3 / 1600x1200 monitor in my arcade cabinet. So I had to convert it somehow, and streaming was one possible way to do that.
Finally, I found a working solution in an external hdmi-to-vga converter, since my projector only has the aforementioned problems only with digital video signals. Luckily, there's no discernable difference in the quality of the analog vga picture, maybe because I spanned the hdmi cable most of the way and plugged the vga converter directly into the projector (it's only the size of a cable's connector without the cable).
Just some experiences that may show you a viable way for your setup.
@BGallagherLA I've just watched your videos . Those are really nice! The main point is you can play "nice and soft" on RBPi4.
If you check my streamings out you'll see the results are the same as yours. But unfortunetely unplayable on RBPi3. See you on Twitch!
Just a heads up to anyone hoping to use this hack when you get round to patching it use the XKAS 0.6 from smwcentral linked in the patches readme and not the one from romhacking was having issues with my initial patch having garbled graphics due to opcode erros turns out the smwcentral one is an unnofficial fixed version which corrects this
thought id give folks a heads up as this is a fantastic hack