You also have to have suspension points(which will be carrying loads under all stresses of maneuvering, i.e.they can't be just anywhere.)PL-15 was developed more or less alongside with J-20. Probably designs were frozen in 2005-2010 period. J20 and pl-15 teams surely had requirements shared to them by PLAAF. So PLAAF could have easily said, "we have this and this bay in j20. to tripack the missiles, we need each missile to be this big and not more"
Basically, by having points arranged like they are in f-22, you sacrifice lots of heavy loads which otherwise(judging by volume) you could use and fit.
Is it a good trade off? For pure air superiority with only secondary strike, it is preferable. But Su-57 has already shown a good example of what heavy fighter can achieve otherwise, and we reliably know about "fat" long range a2a missiles fitting there. It brings an interesting point: The closer "interceptor" (silver bullet) theory to reality, the more feasible retaining 4 points becomes. And otherwise, if j-20 is intended to become "core" fighter, we shall expect 6 missiles.