Granny Beldam
Granny Beldam is an NPC who stands outside of a small shack on a small island in Nerva Archipelago north of Arbiter Frederick and south of Agincourt. He is not a contact, but it appears that he will grant a fifth costume slot as some point. His coordinates are (2692.5, 11.40625, 6050.5).
Currently, Granny Beldam appears as a scientist holding a scroll. It is rumored that the model will be replaced.
When clicked, Granny Beldam says:
So you think you know the season of the witch, eh? Well, during the days of the midnight harvest, you should come across some things that I could use... Halloween costume pieces that I would love to add to my collection. Bring me one Statesman Mask, one Lord Recluse Helmet, one Back Alley Brawler Gloves, and one Hamidon costume. If you are capable of that feat, I will bestow upon you another Costume Slot, just don't make the new outfit as ugly as the one you're wearing now.