As luffy have now 4 special forms (gears) how much they are different, can anyone please explain the differences?
Available on Wikia in detail. I'll try to brief it up a bit.
Using Gears is selecting the right tool for the right job. Each Gear has its own drawbacks and Luffy must also consider the impact it will have on its body. For Example. Gear Four may seem the strongest, but it leaves Luffy completely defenseless after the timelimit.
Gear Second: Luffy uses his legs (Just his hand post-timeskip) as pumps to increase the blood flow in his body. This provides him more oxygen and nutrients. Enhancing Speed and Strength though affecting his endurance to the point of paralysis after prolonged use. Rob Lucci compares its affect to doping. Luffy can use this technique because of his Rubber Blood vessels which don't tear under extreme stress.
Post Time-skip he gains much more control over this Gear, saving his Stamina. He can activate and deactivate it quickly. Used in combination with Haki he also can use much more powerful attacks.
Luffy corks his mouth with his thumb, bites into his thumb joint (making a small opening) and blows very hard into it, which inflates his arm. He is then able to transfer the air through his entire body, requiring the torso to act as an interface for his various limbs. Luffy blows so much air into his body that when they are all focused on a single limb, they become the size equivalent to (or greater than) that of a giant's, if anything resembling Oars' size. Used with his already-strong physiology, this apparently gives his arm more mass and a larger area to attack with far greater strength, but he sacrifices mobility for this strength, as his limbs are now so large that Luffy has trouble moving.
In Gear third he essentially inflates his Bone to Giants (or larger) size. The increase in mass and momentum give him tremendous Raw power (Destroying Marine Battleships, Oars Backbone etc.) Major difference for this is that Luffy loses his elasticity.
Unlike the normal Gomu Gomu no Fusen technique, bullets ricochet off of Gear Third instead of bouncing off.
Post-Timeskip he can compress the air to the end of his limbs for more power.
Gear Third would increase the size of the majority of Luffy's arm, thus reducing the speed of the attack in exchange for a powerful impact. However, Luffy after two years can seemingly control where the air is stored within his limbs, where his fist becomes large and instead of the air being equally distributed throughout his arm, an equivalently large mass forms behind the fist. This allows a majority of Luffy's arm to remain as it is, permitting him to stretch it, thus allowing him to attack and withdraw much faster than pre-timeskip.
Gear Four:Boundman Luffy puffs up his Muscular Structure giving him more power, while keeping up his flexibility. This seems to be a cross between Gear 2 and 3 while utilizing his Haki.
Luffy coats his arm in Busoshoku Haki before biting into his forearm. Similar to how he activates Gear Third, he blows an incredible amount of air into his body, but this time he inflates his muscular structure before distributing the air throughout his body, with emphasis on his upper half.
It achieves to counter both drawbacks of the first 2 Gears
While Luffy is using Gear Fourth, his physical strength and speed are increased to the point where he is able to overwhelm a member of the Shichibukai, Donquixote Doflamingo, whereas before, Gear Second's lack of strength and Gear Third's lack of speed were ineffective in the fight. [...] His durability is also enhanced: despite being hardened with Busoshoku Haki, his body is still rubbery, so physical blows (even ones enhanced with Busoshoku Haki) simply bounce off of him
However, it lacks in range. And thus Luffy needs to be close to use the techniques.
Unlike the previous forms, Gear Fourth focuses on compression more than stretching. While the compression grants Luffy's moves overwhelming speed and power, the range of his attacks is severely decreased.