Dydd's Journal (NWN)

This is the journal of one Dydd, another contestant in Mutamin's Challange. The last entry seems to make reference to the riddle with the numbered doors in the room where his body lies:

Curse this riddle! As near as I can tell the grand daughters that the riddle mentions expand in a pattern. 4 to 5 to 11? What comes next? It isn't 16. That is all that I know.