You are correct. The first of the six available extra triggers is usually used by the 1/4" trigger jack. David, please correct me in I'm wrong, but I believe what he usually does for 'maxed out' Zendrums is wire that jack in parallel with one of the custom triggers.
The pentagonal pad is an unused control.
The Z4 has 33 addressable inputs. In a stock configuration, the first 30 are triggers and the rest are change controllers.
#1 through 24 are the physical trigger pads on the face.
#25 is the trigger jack on the back.
#26 through 30 are unused.
#31 is the sustain switch.
#32 is unused (the pentagonal one).
#33 is the CC jack on the back.
There are technically an additional 5, but they are used internally to support the scroll buttons and such. You know what, forget I even mentioned those.
And congratulations on your new Zendrum! Be sure and post pictures so we can drool over them