But you can post as much as you want. Even 20 per day. The only thing that changes is that posts after the fourth will make less money. So you could easily put all your valuable content in those 4 over each day and then use additional posts for more content.
I think 4 is a good rule, that is enough to write 4 bigger posts but prevents lots of 1-2 small sentences posts and gif posting for the lulz which create lots of noise but bring not much information.
All in all this rule brings that you need to put your information more bundled and compact into the space you have (4 posts) if you` re interested in making money on those posts. Else simply post.
Same for news, i don` t need a single news every 10 minutes, but i would prefer longer news articles with more information.