I guess it's been covered, but there's some kind of unfortunate, crazy idea that 5, 4, 8 are "rare" and thus people mistype as them. I actually don't even get it. I wouldn't want to be an E8, it makes no sense to me. E5 makes some sense, E4 starts getting again outside my ability to relate, even if I can explain it intellectually. It may be that at least 4s and 5s are less common (I'm not really sure 8s are rare, maybe? I don't get why they'd be rarer than CP 6s, which sounds like a pretty counter-intuitive type compared to 8 -- I personally identify more on the phobic end), but seriously, rare =/= what I'd want to be. Honestly I'd rather get what I want from within my type's prism, whether that be 6 or 7, than be a rare type that seems more foreign to me.
I've encountered real 4s and honestly been like wow, interesting, that's foreign.
The real reason a lot of people type as 5-ish or 4-ish is simply that the more trait-theoretic presentations of enneagram present these types as some of the most N-ish, introspective, cerebral types. Plenty of people who join these personality forums have some tendencies in that direction, and thus it's natural that these types get over-saturated, since these trait-theoretic presentations dominate the tests (since that's how a test can even be constructed generally).