Some gear 4 theories:
(1) Gear 4 will see Luffy break the fourth wall and use his rubber body to erase the drawings of his opponents. This makes Sanji and Brooke jealous for obvious reasons.
(2) Gear 4 consists of stretching his stomach - thus Luffy inflates like a puffer fish. He then has the ability to fly unpredictably by releasing air through either end of his digestive system. Beware of the rotating speed-punching fart-burp tornado.
(3) Gear 4 is like the One Piece - the real reward is in the journey. Similarly, gear 4 just means that in a fight, having heart is more important than any technique. Gear 4 means that Luffy is now fighting with his heart. [Disclaimer: of course the One Piece is real]
(4) Gear 4 makes use of the stretching heart to pump blood at incredible speeds. This results high-pressure nose bleeds can cut through concrete. What's more, the more you cut Luffy, the more he bleeds on you, cutting you to pieces like a lightsaber disco ball.
Combinations may apply - imagine (2) plus (4)! In that case Gear 4 could just be total inner-body control and the resulting techniques.