Gear 4:
Gear 4 is a very, very impressive gear. It doesn't look bad when in it is in action.
Of course, this is Gear 4's chapter, it was the title. People seem to jump straight to Gear 4 > Dofla. It isn't the case. What is the case however is that the Gear 4 is very versatile and we have only the tip of the iceberg as to what it can do.
IT WILL NOT DOWNPLAY GEAR 2. Gear 2 still has its purpose, and that is to be manoeuvrable. Luffy has a ton of speed but not as much power, Luffy will also be able to move normally.
IT WILL NOT DOWNPLAY GEAR 3. Gear 3 is the destructive large scale attack used for destruction e.g. Noah.
How does Gear 4's attacks work? It's focussed on concentrated or redirected power.
The Python attack is simply a Geppou, By Luffy having his punch hit the air hard enough, it will bounce of it. This has the same mechanics as the Geppou. Luffy uses his Haki will still remaining Rubber and direct his Python through Geppou.
The reason why Doflamingo was asking why the attack did not retract is because Luffy is rubber, and naturally it would retract. However in this case, the Geppou used to redirect the punches maintains the punches original strength whilst bouncing off the air not resulting in a contraction.
Gear 4's Defense. Gear 4 is also a defensive ability with the aid of Haki.
The reason is because it NEEDS to be, Gear 4 is actually not as manoeuvrable as we might see. Luffy can't stand still and keeps bouncing.
Doflamingo mentioned this and said "you can barely stand on your feet".
Luffy moves in a straight line. Luffy in Gear 4 directs his "geppou" and charges in that direction. You cannot control a bounce and make it move by will. Naturally it would have to bounce off something and it will go in ONE DIRECTION
This is shown in the chapter as Luffy charges in one direction, even when Doflamingo flies into the sky.
This is Luffy's Drawback but it's also where the defence side comes in. Even Though Luffy's attacks are powerful in Gear 4 he lacks manoeuvrability as he can only bounce in one direction. Thus needing a defence capability
Doflamingo displayed how Luffy was wide open by attacking Luffy with the Hamstring. However, this was going to be the case being a bounce man, as it causes a loss in manoeuvrability This is why Gear 4 won't leave Gear second out. GEAR SECOND IS NOT JUST SORU, THERE'S A LOT MORE TO IT AND DOES SERVE A CRUCIAL ROLE.
So to compensate for this, Luffy uses Haki and the rubbers properties to be able to bounce off attacks unscathed. This had to be the case since Luffy is going to be wide open at times, so it has to support him defensively.
Whilst it's not as destructive and Massive as Gear third, It is less powerful with a much lessened AOE as it is concentrated. But unlike Gear third, Luffy can afford to be less manoeuvrable as he has that beastly defence with Rubber + Haki.
Whereas Luffy in Gear third cannot afford to be non manoeuvrable as he would get tossed around and be countered easily as displayed Last chapter.
This chapter didn't touch on the Drawbacks yet for plot purposes. However Doflamingos displayed that he can easily counter it, but needs to change his plan of attack.
The drawbacks don't necessarily need to be "exhausting" but fight costly such as manoeuvrability but can be compensated with the use of a defensive perk
Luffy will NOT rely on Gear 4 all the time.
As this is Gear 4's first appearance, there was a slow activation to show what was going on (just like the first transformations to Super Saiyan).
Luffy will be Switching from Gears 2-4 efficiently as the story progresses. Whether or not he will in this fight, I am not sure.
I'll post in G4 Thread as well.