I have both. My answer largely depends on the pack you use because I'm assuming from your post that you are just wanting to compare frames and the pack is irrelevant. I think Kifaru makes one of the best frames on the market but I have never experimented with using other bags on that frame other than a couple of Kifaru bags I own. I do attach a customized lone wolf on the Ark when its deer season and I'm packing in a climber. The K4 I use with the K4 pack as I bought them together. Frame wise the Kifaru fits me like a glove. The K4 is comfortable but the fit isn't as snug. I'm 6'1" 184 lbs for reference.
My answer largely depends on the hunt as I'm not a massive fan of Kifaru's more expedition like bags. Because of that, if I'm packing in gear, tent, weapon, and all my food in an expedition type hunt, I'd recommend Exo K4 pack and frame. If I need a customized pack set up where my pack would be used in a variety of ways, I'd recommend the Ark. Exo K4 for me is the Keep it Simple Stupid method when I'm going a long time and need to carry a lot. The Ark usually requires a lot more technical adjustments to make whatever is on the pack comfortable, especially when you are strapping other things to it. You just notice immediately if its not balanced correctly. The k4 in that regard has a lot more user error built in.
Kifaru's frame can stand alone with many pack set ups, but I just haven't experimented enough with a lot of bigger expedition packs with that frame other than one which I was not a fan of.
Hope that helps.