Any cylinder will do!!!!
I don't think that is correct. Number one can be on either the compression or exhaust stroke but it needs to be on one or the other. I don't think timing it with number three or four at TDC will work. It may work with number two at TDC but why do that. Just put Number one at TDC and blow in the tube. I may be wrong but if you could time it on any cylinder, then why would you have two coil banks? I'd either check deeper into this or just use number one to time your PMags. Good luck.
It seems everyone is correct, as long as it is at TDC for any one of the cylinders you are good to go, We retimed the mags during troubleshooting this weekend after I had my first flight adventure on saturday and this was the main questions that came up, does it matter which. The answer is No it does not matter.