Super Saiyan said:
Except we do know how far apart the ships were.
We've also seen how far Base Luffy can stretch his arms.
That image isn't so good to quantify the distance as it's rather zoomed out. We can't infere much from the Thousand Sunny's perspective.
Stretching his arms when trying to reach the enemy ship would also lead to them becoming more vulnerable and he has already seen how a moment of carelessness in this aspect can lead to it being slashed, such as in the fight against Cracker.
Captain Cadaver said:
Using your most taxing form when needing to conserve energy for an extremely important battle is inexcusably reckless
We don't know if it's still the most taxing form. Maybe it's being used so casually because it no longer is, or if it still is, it still compensates due to its ability to fly around.
This is further damaged by Luffy's recklessness being the entire reason he got beaten by Kaido and something he's been criticised for throughout the entire New World events, yet hasn't taken even one stop in trying a more rational and conservative approach to battle.
Yeah, he's impulsive, I can give it in. But on the other side, weren't for it he maybe wouldn't have encountered that Hyogoro old man and wouldn't have advanced on his training, as the whole prison segment was important in this aspect.
The fact still remains that, from what we're shown, he didn't need Gear 4th to bridge the distance between the boats.
Maybe... we don't know. See my point up above.
We're already into the climactic conflict of the arc, so leaving it this far to offer any explanation for a clear contradiction to the rules of Gear 4th is poor storytelling that can only result in a sloppy explanation.
Compare this to something like Goku mastering Super Saiyan. In that situation, we're given a clear incentive of what he needs to accomplish, see his process of trial and error in training and then see the result. Assuming Luffy's training was to master Gear 4th, Oda completely skipped the 2nd step and, thus, made any sort of impact from the result seem hollow.
I really see no difference in a piece of information being explained to us during the occurring of the scene or after it, in which case, Luffy's main battles are a good way to expose to us in higher details the nature of his training. We aren't still that advanced on the arc, either. It's reaching the conflict, which is the bulky of the arc and according to Oda an event that outclasses even Marineford in importance, so there's space for a fine detailment to be made.
We're specifically told that using it so much will shave off more of Luffy's lifespan and Nami even says this isn't something Luffy can keep using in excess after Doflamingo's defeat, so that wouldn't work. Even if that was the case, again, Oda skipped over the step of showing Luffy's training and new methods in mastering the form to mitigate the weakness that lessened any impact of seeing him perfect its usage.
Sure, though G2 was also presented initially as very taxing to the body, to the point he could maintain it against Lucci with difficulties, with Luffy having to push himself to it after the first usage, yet he's apparently more used to it by the time of Impel Down and Marineford... I assume the heavy usage made Luffy's body more used to it in a way, similarly to how the SSJ is presented as restless on the body and said to be life-consuming and yet Goku's solution was keeping the state activated almost 24/7. It will have lasting consequences, and they'll likely feel it later on in their lives, but that doesn't mean the body can't momentarily adapt to its usage.
Think of someone smoking for the 1st time and coughing heavily and having a sore throat because of it... but if you push this habit to you, your body will momentarily adapt to it, at the expense of lasting consequences later on, though. The body can adapt to a restless thing while accumulating the harm it leads it to with time.