The 204 is a neat little cartridge. However if I was going to buy a complete upper I would probably go with the 223/5.56 cal. There are a couple of reasons. One being the availability of the ammo anywhere as well as the cost. The other is regarding cost of the 204 would only be best if you reload everything you shoot. Finally the availability of various grains of bullets. I have shot coyotes, fox, bobcats and pigs with the 223/5.56 successfully by selecting the correct bullet. Unless the 204 had perfect shot placement it might be a stretch to bring a wild pig down. Sometimes it is a little bit of a stretch for the 223/5.56. The other issue is you stated the 22" barrel length. I might assume you were referring to a bull or heavy barrel. You want to rethink that when considering walking around all day packing a heavy rifle looking for yotes. I might suggest that Rock River makes a Predator Model and a Coyote Model which both are 223/5.56 uppers. The Predator Model uses the same lighter 20" Stainless barrel that is used at Camp Perry for the National Match Rifles and it is extremely accurate. And I agree with the AXX man!
Even though 600 yard shots can be made with the 223/5.56 it also is greatly effected by the wind. And the 204 even more. I would not suggest shooting a varmint at 600 because even though they are predators I like good humane kills on anything I shoot all the time.
That's my take on it. Good luck on whatever choice you make.