|Problems? Is your data what you think it is?
Relax. It's okay for people to get to level 4 quickly when they actually post decent stuff, because that sort of positive reinforcement encourages decency (very effectively, I think). The jump from 5 to 6 (from XP200 to XP500) would, on average, take at least as long as the whole progression from newbie to level 5, and so it's not surprising to see a clump of folks who hit a "plateau" there.
As for the gradual but inevitable growth of the saint caste, if PM were a for-profit corporation and our saints were managers and chief officers, this sort of growth would be bad. But since PM is an all-volunteer, all-pro-bono, altruistic mob, having more people at the highest level just makes things better for everybody at all levels (even for those levels less populated than the saint class) -- it means more experts to share the SOPW load, more diversity of expertise, better jokes in the CB, you name it.
If/when I reach that level, it'll be meaningful for me, no matter how many others are there by that time, no matter how many more come in afterwards.
update: Having thought a bit more (and looking at some other replies), I wanted to stress this one point about XP and levels: it should not be about climbing into ever more rarified and exclusive upper ranks. It's about accumulating a common wealth of experience, such that people are accountable (get credit) for the experience they contribute. A lot of effort is invested to make sure the wealth can be shared as openly and effectively as possible. When there are more saints, the whole community is richer, and I don't see a problem with that.