Luffy has haki. He's not using G4 for the same reason Zoro isn't using Santoryu, both are conserving their strength/stamina.
The idea that Kaido is playing around with the Supernova or otherwise not seriously trying to kill them is alternative history.
At the end of Chapter 1001, the Yonkou declared their intent to kill the Supernova and that doing so shall place them "that much closer" to becoming the Pirate King.
Where was the Yonkou's supposed attitude of playing with their food when Mama sneak attacked Luffy while he was recovering from Kaido's Thunder Bagua?
Where was the Yonkou's supposed attitude of playing with their food when Kaido did the same?
Where was the Yonkou's supposed attitude of playing with their food when Mama sneak attacked Killer with Indra:
Or when she did the same to Zoro?
Where was the Yonkou's supposed attitude of playing with their food when Kaido tried to chomp on Killer while he was temporarily out of commission?
Where was the Yonkou's supposed attitude of playing with their food when Mama tried to behead Luffy while he was immobile?
Or when she tried again?
Or when Kaido tried to chomp on the immobile Luffy?
The Yonkou have been seriously trying to kill the Supernova. Just because they haven't succeeded doesn't mean you can claim they were only playing with their food. If they were indeed just playing around, they would never have resorted to sneak attacks or trying to catch the SN off guard. They wouldn't have relentlessly gone after Luffy when he was unable to defend himself.
Hell, most of the Yonkou's attacks on the Supernova have been on a Supernova that was:
- Off guard, disoriented
- Mama's Heavenly Fire at post-Thunder Bagua Luffy
- Kaido's club swing at post-Thunder Bagua Luffy
- Out of commission
- Kaido trying to chomp Killer after Mama temporarily KOed him with Indra
- Immobile
- Mama's Motherly Fire at post-Boundman Luffy
- Mama's off panel attack at post-Boundman Luffy
- Kaido chomping on post-Boundman Luffy
- Sneak attacks
- Mama's Indra at Killer while he was focused on Kaido.
- Mama's Indra at Zoro while he was focused on Kaido.
That's not the attitude of people that are playing with their opponents or otherwise trying to prolong their suffering. The Yonkou were dead serious while trying to kill the Supernova. They were just simply unable to defeat them with the level of strength they had been using. As a result, they powered up (Kaido used his Hybrid form, Mama used all her three homies and fused with Prometheus); they were still unable to kill the Supernova.
The idea that the Yonkou were not trying to kill them since they powered up is just bad fanfiction — the Yonkou were simply not strong enough to do so.