Those "extra" 3 or 4 cm of seatpost that you'll have exposed will come in handy for the stoker's stem, etc.
Many of the photos posted for this group show a lot more captain seatpost than you'd see on a road bike, even in these days of relatively small frames (contrasting to days of yore when you got a frame as tall as you could stand over).
A hair more standover is welcome when starting up using "the proper method", too.