The reason was the result of voting in Torvalds Google+

Linus Torvalds decided that the next version of the Linux kernel should be 4.0. He went to something like that according to the results of a poll in a post on his Google+ account, where Linus actively maintains some semblance of a blog.
On February 13, Linus, in his characteristic frivolous manner,
put to the vote the important issue of a free operating system. The situation began to resemble the situation with version 2.6.39, after which Linux 3.0 followed, as Linus was tired of distinguishing these large numbers. This time, as the author of Linux put it, he began to run out of fingers and toes for the account. Talk about fours
began in 2013.
A seemingly half-amusing poll with two answers was attached to the usual
post : “v4.0, because I get confused” and “I will not lie, I love the big versions” - a reference to the song
Baby Got Back , which dealt with women’s hips . In total, more than 30 thousand people voted, and the distribution was 56% to 44%, respectively. Nothing unexpected happened during the voting, the ratio
remained so from the very beginning.
On Sunday, after a “comprehensive statistical analysis of my G + poll,” Linus
released the release candidate for Linux 4.0-rc1. The code name of the new version was also put to the vote, and Hurr durr I'ma sheep (“Hurr Durr ya sheep”) won “I love online surveys” with a ratio of 62% to 38%. About 6 thousand people took part in the survey. This is not the strangest name, Linus has long been fond of shocking. Examples are Carnivorous Bats with Fangs (Linux 2.6 branch), Sheep on Meth (Linux 6.34 / 2.6.35), imitation of a pirate accent in Linux 2.6.23 ~ 2.6.24 (Arr Matey ! A Hairy Bilge Rat!) And Ermine in Fat (Greased Weasel, Linux 1.3). Wikipedia has a whole
article with a list of such oddities .
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Thus, Linus almost completely neglects the
semantic version numbers , in which the first number signals incompatible major changes in the API, the second - the minor additions of backward-compatible functions, and the third - the patches of backward-compatible fixes. Google Chrome and Mozilla Firefox, the most popular browsers, ignore this system, the latter released version 36 yesterday.
The new version has
many changes . The main changes are improvements and file systems OverlayFS, XFS, Btrfs and F2FS, support for IBM z13, many new ARM platforms and apply patches to a working kernel without rebooting the system, improving sound, media drivers, multiple x86 KVM optimizations and so on.