I still have no idea if Oda wants to bother with an Act 4 but we did have Act 1 curtains closing on us before we went around the world before Act 2
Imo, it has to happen. Act breaks seem to occur with timeskips and changes in arc events. So an Act 4 should definitely be around and contain:
- Road Poneglyph stuff
- Lore (Zoro/Lineage/Ryuma stuff, Other things)
- Big Party
- What many characters intend to do
Hell, there may be an Act 5 (swift conclusion) that's like 3-4 chapters. Of the crew leaving Wano and closing threads.