Full Bordello 4 timeline goes like this:
January 2023 - Bordello 3: Lyria is released, mentions ideas for Bordello 5 (not 4) and that he's planned the Bordello series out quite far
February 2023 - Officially starts preproduction on B4, shares pinup of new featured character Rose
March 2023 - Reveals that Calarel will be the star, shares a bunch of renders of planned sex scenes, as well as environmental and object renders, says he's aiming for a shorter runtime than B3, "montage style" with more positions but less variations
June 2023 - first animated preview of B4 (facial)
November 2023 - second animated preview (kissing/thighjob)
December 2023 - third animated preview (mating press), mentions some camera angles might get changed in the final release
February 2024 - Story preview, some narrative-focused shots
April 2024 - Mentions in Worshipping Selune release that he is going all-in on finishing Bordello 4, no more animation side projects besides the usual pinups and any animation challenges
So it's been almost a year and a half. My theory on what has been delaying it so much is that he is really trying to do as much of the modeling work as he can by himself. He's sculpting all the props, making the environments, all that stuff, and they seem to be pretty intricate and detailed. He mentions having help with the new clothes being made for Calarel and Darcey, but I think that might be it. Seems time consuming. That and I'll bet that it'll end up being longer than B3, even though he said he was aiming for it not to be. Maybe it's just copium but I feel like something taking this long, assuming it was consistently being worked on, should not be less than B3's 10 minutes.