@STP_PS:
There are certain systems that have not been explored and that could do with gear 4.
1)Muscles (as stated by many which to be honest makes sense somewhat)
2) The Nervous system. Now this I find interesting, All movement related to any being involves a bioelectric signal. Now, Luffy being a rubber man, has bioelectric signals that travel through rubber. Which either means that these signals are also different, from your average fellow or that somehow his body manages to carry on signals via other means.Could he excite his nervous system so as to reach an impact rupture and cause a wave (such as the impact dial or the Happou navy technique) who knows but it sounds quite amazing.
3) The lymphatic system. Which is the system that has to do with removing dead cells, debris, patogens, toxins and destroying waste. I[m not sure how this system works in a rubber man, but what would happen if Luffy could somehow alter it via air inyeccion, compresion or other. Who knows, even if I don[t believe that Luffy will ever get some regenerating ability, maybe his endurance and durability could be enhanced by altering and playing with this system. It is also the system that helps to process the blood through capilar filtration. so theres a lot to explore here.
I am sure there are more systems in human biology but the ones I can think of considering Luffy's last stance (reaching his wrist to his mouth) are these. Then again, it could be related to skin for all we know.
I like all three of your suggestions.
Luffy bites his finger and blows air into his bones for Gear 3. Maybe for Gear 4 he bites his wrist and raises his body's carbon dioxide levels. There would be negative effects to this when a normal person does this but maybe using One Piece Logic it could result in a power up, similar to how Gear 2 and 3 works out.
Carbon dioxide..
Helps dilate smooth muscle tissue.
Helps regulate cardiovascular system.
Higher more active tissues and cells generates higher levels of CO2 resulting in immediate local vasodilation (relaxation of smooth muscles with the result of increasing the diameter of the vessels), thus setting the stage for supplying the required oxygen and glucose to the associated tissues.
This would take into account what I said about Luffy's respiratory system being the limiting factor…in the end might help him greatly in surpassing his limitations.
I am also thinking that if the level of CO2 increases Luffy might turn blue similar to how he looked in his Nightmare Luffy form. Gear 2 Luffy might be overbreathing which might result in having the most amount of O2 his body can handle..causing him to turn pink/red. If Gear 4 results in hims having the least amount of O2 his body can handle he might turn dark blue.
Still hope to see some electricity and what I said about the nervous system previously. However, I personally don't mind if Oda brings back a different variant of Nightmare Luffy instead...