You won't gain anything by putting a Gator on the rear and a 4000 on the front, the Gator will slow you down a bit like both are doing now. Actually if you're concerned about flats then get the 4000's for front and rear then on the rear add a Panaracer FlatAway liner which is very tough and it's the lightest weight liner on the market.

Personally I wouldn't even use 4000's but I know how everyone thinks Conti are the bees knees of tires, but in reality they're not, there are other tires on the market that are at least as good if not better for less money.