And it does not mean Luffy is holding back if he didn't use G4. I think for someone like kaido and big mom you can't just use and exert everything you have on them from the start. When they have no idea if that will work or not so going all out will weaken someone's strength.
It was best explained by Zoro in chapter 1010.
Which means that they can keep going and exhaust all of their stamina. Zoro chose to not waste more stamina than he already did and use up all of his remaining strength instead of slowly getting weaker for nothing when their opponent seems fine.
Luffy did the same thing earlier when he chose to burst all of what he got on Kaido but he failed and Zoro had to look over his shoulder for Luffy's blunder.
The fights in one piece is usually organized and easy to read.