GEAR 4 = FULL BODY Muscular Vulcanization Theory
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What is Vulcanization?
Vulcaniziation is a process that hardens rubber. Examples include tires, shoes, bowling balls, etc.
Gear 2nd involves pumping blood faster, and Gear third involves inflating bones.
Gear second =
Speed (through blood pressure)
Gear third =
Strength (through bones)
Gear fourth = vulcanized muscles (through vulcanized muscles) resulting in great strength while still retaining speed
This is how luffy starts red hawk. Now imagine his entire body like this for the pre-transformation.
Imagine an explosion of fire from luffy's body resulting in Gear 4 once the smoke clears.
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Rubber can only vulcanize through extreme heat. We know luffy can produce fire through Red Hawk.
Red Hawk is a result of the friction of gear second combined with the hardening of armament haki. Through that method, Luffy will cast an explosive Red Hawk over his entire body to vulcanize his muscles. After the smoke clears, the flames will also dissapear; resulting in an
appearance similar to Nightmare luffy.
Full Body armament haki + Full Body Red Hawk = Vulcanized Rubber and increased muscle mass
Through heat vulcanization, his muscle mass will increase due to hardened rubber. The flames will dissapear and the result is a muscular luffy.
Note: This is not the same as full body armement haki. Luffy's armement haki does not increase muscle mass. However, he will use armament haki combined with intense heat to create vulcanized muscles.
His body will be even more hardned than through normal armament haki. Rubber becomes extremly solid due to vulcanization. For example, look at a vulcanzed bowling ball and its density
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His skin could even be blue just like nightmare luffy due to lack of oxygen. This is because muscles require even more oxygen than normal. Therefore, his muscles will take a huge amount of oxygen from the blood.
If you disagree, look up blue blood and its connection to lack of oxygen.
Weaknesses of Gear 4:
1) The main weakness is the lack of oxygen to the brain, which means luffy won't be able to stay in the form for too long without passing out. However, I'm sure he will overcome this weakness in the future just like how he overcame the weaknesses of gear send and third.
2) The minor weakness is decreased stretchiness due to vulcanized rubber
Luffy gains access to the Soru technique when he utilizes his Gear Second More, which pumps up his metabolism and allows him to move at disappearing speeds.
Also, Sanji acquires one of the CP9 Techniques called Moon Walk
Again, Luffy might also take another CP9 technique - The Rokuogan (the ultimate technique that Rob Lucci used)
The Rokuogan was an attack that send shockwaves into the opponents body.
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Since luffy learned Soru, he might he have trained to use an attack similar to the Rokuogan during the timeskip.
His
increased strength with Gear 4 could allow him to punch through the air, as he stretches, to create shockwaves.
It would make up for the decreased stretchiness, and thus eliminating the long-range weakness. This technique would be inspired by the Rokuogan. It would be the ultimate technique for Gear 4
All if his former attacks could be used to produce shockwaves to hit foes at a distance.
They would be similar to the shockwaves that Burgess can produce with his surge elbows. Im sure luffy could do the same with his Gear 4 strength and muscle mass.
Conclusion: I believe Gear 4 will be a very similar to Nightmare luffy through rubber vulcanization.
His muscle mass will be greatly increased and we might even see shockwave attacks based on Lucci's Rokuogan
~creadits to the owner
Hiatus 
