kegging less than 5 gallons

I am going to keg a birthday beer tomorrow but am unsure of how to prime. I am anticipating 4 gallons siphoned to the keg and want to naturally carbonate. So, question is, how much sugar should I boil and prime with? I plan on purging with co2 and could transfer under co2 pressure with cane and co2 hooked to cap but plan on using gravity to fill. I could force carb, but like the idea of using sugar. It does not have to be ready until the 31st but is ready to leave the yeast cake.(primary for 4 weeks saturday) Any input on this would be great. From now on the batches will fill the cornys since I now brew 10 gallons at a time. Thanks for your input.

I'd say use this priming calculator and then subtract 20%. It's much easier to up the carbonation a little for an undercarbed beer than it is to de-gas an overcarbed beer. Also, a lot of people around here claim that the same amount of priming sugar produces more carbonation in the keg than it does in bottles. That hasn't been my experience, but maybe I'm just doing something differently.