I've been seeing a lot of comments from John Mueller recently, mostly via SE Roundtable, suggesting that there are a lot of things you can't tell about a page by what you see in Google cache. Cache dates, eg, don't really relate to when page was last spidered, etc etc. .
These reported "problems" have not seemed to be connected... but a lot of them do seem to be happening around now, which suggests perhaps that Google's changing something in its infrastructure....
The particular problem you have described, of cache 404 errors, was recently addressed by John Mueller in this seroundtable post...
Google Cache 404 Error Does Not Impact Your Search Rankings Apr 12, 2018 - by Barry Schwartz [
seroundtable.com...]
John Mueller from Google said on Reddit that if your cache link in Google returns a 404 error, that it doesn't mean your rankings will be hurt in Google. John said "that can happen, it doesn't mean anything."...
.... The Google Cache 404ing is an internal issue with the Google Cache server and doesn't reflect on indexing or ranking of the web page.
It could be something that's happening in relation to mobile-first. That's as good a guess as any. Chances are the Google may not comment on it more specifically unless the problem gets extremely widespread.
The cache server is separate from the ranking server, and I'm guessing that there are so many variables (not effecting ranking) that Google doesn't feel that a more precise answer is needed. It doesn't matter. In general, if traffic isn't affected, I wouldn't worry about it.
