I think it's been foreshadowed by mingo. Mingo said that luffy has speed but not power. And when luffy used "Elephant Magnum". Mingo said he had a blind spot because of the hassel of the weight of his giant 3rd gear arms. Now what I think Luffy's gear fourth would be is. both of luffy's forearms will get bigger but not to the point of gear 3rd. just about the size of bowling balls and will have arnament haki imbued with it. So his power will increase as well as not slowing him down not near as much. I don't know if luffy will have arnament haki on his entire body but if so. I could see his shins (from knee to ankles) being slightly bigger as well to add power to his kicks. Also I can see him being slightly faster than normal in this mode. What do you think?