I unironically believe the hopium that we're getting more than 1 act. The weapon in the teaser is from the final boss of act 4. Sure, the "charging" or whatever Sin does might be the final moments of act4, but then I really doubt GGG would use it as the teaser.

I think we're getting act 4 where we hunt the weapon to kill the best then we go to act 5 and charge it, and potentially already fight the beast once. This would also line up with Jonathans recent comment that "cruel will be gone sooner than we think" because if we had 5 acts it would be easy for them to add a small bit of filler content to get us ready for mapping.