Jonui's theory from last chapter was a good call. Baku does not bet on luck; it is more in character for him to believe in Hal and fold here, continuing with the original plan. We as readers know Fukurou made an invalid hand, but no one else in the game other than Shion does. Betting everything on the chance of a miss from Fukurou wouldn't make sense, unless he had somehow intentionally made him miss.
The question now is what hand did Hal make, and where does that leave us for Round 5? I think most of us are expecting Hal to have made a junk hand here, so they can use 25 in the final round. Hal making a dumb hand would certainly be shocking to the referees as well. Do Fukurou's other cards remain in the pool, or are they all removed (I believe he loses the 4 "new" cards he chose)? If Fukurou picked the wrong suit of King, doesn't that mean there's still a King in play, since the King of Diamonds hasn't been selected and removed? Does that change either of the remaining numbers? What about Hal's cards? Does Hal have to suffer the witch pain because his partner folded, or is that purely based off the numbers?
If its the latter, I think Hal and Baku might have made Fukurou miss somehow. If the Witch Pain punishment is purely based off of the hands Fukurou and Hal make, and we know an invalid hand is considered a "loss", maybe he has to experience the Witch Pain even if Lalo wins below. Hal is insistent that another hit of the Witch Pain would break him, perhaps they made Fukurou miss so they could both lose the hand (and preserve 25) but allow Hal to escape the punishment so he can think clearly for the final hand.