Benny I thought about the same but here's where my issue is; you certainly can cook all four meats but whether or not they will be at a comp level is questionable.
IMO, you will benefit by having different cooking environments for the differents meats.
For example, brisket can be cooked in the low 300s, shoulder in the low to mid 200s, ribs in the upper 200s and chicken in the mid 300s. If using a WSM and the shoulder is cooking slow you can easily bump up the heat. With a RK you can't do that without impacting the other meats.
I hope I don't discourage you, if you can manage to put out quality comp meats on a RK my hat's off to you, it's not easy to do.
I think it's more ideal to use at least 2 maybe 3 WSMs or a kettle and then the new 22 when it comes out.
1 18 WSM pork
2 18 WSM brisket
3 18 WSM or 22 kettle chicken or use one of the above when meat is done
4 22 WSM ribs
John