While I can't really answer your question, as it is very specific; hopefully something is a bit helpful in my response.
I remember back in the day, we used to do audio editing on a Netbook, using live instruments and input/output samples, using an Atom N2600 CPU, and we could get latency down to about 128samples, or 12ms. The delay was barely noticeable, but overall not too bad.
We later upgraded to a Celeron N3060 with 2,48Ghz boost, and latency was well within 48 samples, or ~7ms (instantaneous); all using an ASIO4all driver, as the windows drivers sucked. It was about as good as my older Core2Duo CPU running at 1.77Ghz
The Pi 4B only has a 1.5Ghz CPU, which can be overclocked to 1,6Ghz safely (provided you have active cooling).
But Linux has generally a very low latency (lower than Windows, but perhaps not lower when using an ASIO driver).
ARM generally has very low latency. Much lower than x86, if made well.
So if you ask me, I think what you're asking is how low can one go, before the sound starts to noticeably have a delay?
If the Pi 4 was running at 1,8Ghz, I'd presume the audio would have very low latency. But a stock Pi4, and via the GPIO pins?
Hard to say.
If you don't want to cash out the money, try getting a used Pi 3B+ somewhere, and test it out, as the Pi 3B+ can be overclocked to 1,5Ghz (with cooling) and isn't that much different from the Pi 4 (save for having slower RAM).
If it works on the Pi 3B+, it'll work on the Pi 4, with maybe 2-4ms lower latency.