According to the docs, which I can't link due to a crappy android keyboard error, GPIO boot Mode is only supported on previous versions of the Pi, not the 4. I can't tell if the documentation is out of date, or if that feature really has been removed from the Pi 4 line. I hope it's still possible, as GPIO boot is critical to the design I'm aiming for. Basically, it allows a hardware device to tell the Pi what to boot from, using GPIO pins. This would allow a game cartridge to tell the Pi to boot from usb or SPI. Booting from SPI is on hold as well. Dead end right now. GPIO boot is on hold as well. I don't have a Pi 4 I'm willing to modify to try and get it working, so I'm once again waiting on more hardware, with no budget for it. The lack of knowledge, funds, and support on this project are making it very difficult. I'm still working on making a bootable OS that loads a game immediately. I can make some progress there, but that's about it until I buy more hardware.