Ulgu. Not only Malerion is the only god that hasn't made a move yet (Tyrion too, but Lumineth are his people and they are very active), but also because I noticed that WHU seasons usually align with the realm focus in the AoS editions that follow them
WHU Shadespire - Shyish in AoS 2
WHU Beastgrave - Ghur in AoS 3
WHU Harrowdeep - Ulgu in AoS 4?
The only thing that makes me think this has changed is that we're back in Ghur in WHU righ now.