@Greg, I don’t think an act-ending cliffhanger would affect the narrative that much. It’s a pretty common plot-device to leave the audience hanging and then pick it up again later. Oda has done it many times before, with flashbacks cutting off action moments for an entire volume (Robin’s, in particular, comes to mind). An intermission after a cliffhanger wouldn’t be as long as that, and it could weave plot points that actually affect the following act.
There’s two main reasons I think there will be five acts: the length of act 3 if there’s no act 4 (I don’t know about kabuki, but in a 3-act western play the second act is the largest one), and Oda expecting to show us something about Hancock, Sabo and Vivi in 2020 ( and he announced it after chapter 956, so it couldn’t be that).
Those, and not a YouTuber’s prediction, are what I base off my own assumption.
If I’m right, Act 3 should end pretty soon, within around 8 chapters, I guess.
If we have our big tens moment and proceed to the real match-ups without and act closing, thiught, I’ll be convinced there’s no Act 4.