Quoted from mrbanjo:R-20-4 isn't even a Gottlieb number is it?
It is:
http://www.marcospecialties.com/pinball-parts/R-20-4
http://www.pbresource.com/coilgtb.html
but, what does the schematic say should be used there? One trick is the sometimes the coils are listed in order on the schematic. The other thing you can do is figure it out by elimination. Cross off the relays you know. Narrow it down based off of switch type and number/type of switches from the ones left.
Or...ask for a picture.