If you don't work at Amazon, there's a pretty big sampling bias in your data. (You are wildly more likely to work with someone who has left Amazon than someone who has happily stayed there, and in casual interactions, you're more likely to hear from the strongly disgruntled than the happy. In the same way, current Amazon workers are likely to know a higher than average percentage of long-tenured and happy Amazon employees than is reality in overall population.)

I don't rub shoulders with BigCo devs, but of all the SmallCo devs I know, the only ones that reach 4 years tenure are the ones that own the company.

Is it unreasonable to want to try something new after 4 years? I don't really imagine staying with any company for more that 4-5 years.