How many pins are required to connect a prusa mk3s cable to raspberry pi 3B+ via direct connection

I'm connecting my raspberry pi 3+ to a prusa mk3s via the built-in header. In doing some research I found a reference that said it can be run with just 4 wires connected +5, Gnd, RX, TX. The assertion was that gp22 & gp23 are not used but were there for stability when using a pi zero. The two pins connected to gp22 & gp23 are tied to reset and ground respectively on the Einsy board. I want to minimize unnecessary connections between the pi and the mk3s. What pins are actually used by octoprint to communicate to the mk3s?

I'm actually much in the same boat you are. The connection is established via those 4 cables, but technically the 5V isn't necessary if the Pi is getting power from an alternate source like an AC adapter. If those two pins are connected to Reset and Ground, I bet my bottom dollar that some plugin for Prusa's Pi Zero image shorts those pins, resetting the RAMBo even if it's unresponsive. How they do it through a plugin or other methods is entirely unclear and would probably require reverse-engineering the Pi Zero image Prusa provides in order to figure it out.

TL:DR; only the 4 pins at the top are necessary, the unplugged ones are probably supposed to reset the board, but how Prusa's image for Pi Zero does this is unknown.

Thanks. I'll be using the 4 wire cable. I want to power the Pi from the MK3S Einsy interface to keep things clean and only require a single power cord to the mk3s.

Be aware the pi may not get enough power from the Prusa board, since it was only designed for the (underpowered) pi zero, which has a much lower power requirement. You may end up powering it separately. If you run into issues this will be the first question.