We have a couple of these in the office that we can run software / test some things on - so what would you like to see tested on the hardware that would help to swing the decision for you as to whether or not it's worth it?
I've seen a few questions about Sega Rally running on MAME, questions about h264 encoding, realtime transcoding - for some of these I'd have to find the software/setup but it's certainly do-able.
So let me know, what do you want to see tested on the Pi 4?
I think there maybe was a hard 250MHz physical I/O ceiling, because on an old Pi (I think Pi 2), in C code, with the WiringPi library (which offers an Arduino-like API) I was able to get a signal of approx 5.8MHz by toggling a pin up and down using the digitalWrite command (and compared it with Python etc here: Raspberry Pi GPIO Explained ) but the jitter was on certain discrete boundaries, i.e. the I/O seems to be clocked out at some rate of 250MHz, too fine to be from just usual context switching. I couldn't find any documentation at the time to support or un-support this theory though.
