I believe that if a gear fourth is trully out there,it wont be related to haki,haki does not grant any abnormal physical aspect,it surrounds,as explained,the body in one's materialized will,or something of that sort,so I dont think haki will directly take part in a new gear other than to maximize it's effect, the principle behind it will rest on the rubber itself other than haki.
I mean, most people say they can't think of a new gear, that's only because they're not willing to put a stain on such creative gears that Oda made,but the possibilities are, indeed, endless. Nobody would think of gear 2,it's something completely out of the box, for a fourth gear Oda could invest on a million different aspects of rubber and make something remotely plausible, the possibility rests only on Oda's predisposition to continue the gears,or move on to a new set of techniques that are not directly related to the fruit itself.
Da Z