On El Capitan there is iBooks 1.5... Where do we get iBooks 4.0 or later??

In the article below, they say syncing epubs etc works in iBooks 4.x and later. I am on 10.11.4 and I have iBooks 1.5. Where do I get iBooks 4.0 or later?

Because syncing ePubs across devices doesn't seem to work for me. I tried each and any hint on the communities or the web, but still there are lots of books missing.

I don't find a pattern. I even created a pdf from Pages on one of my Macs, imported it into iBooks and it doesn't sync... So it doesn't seem to be a DRM issue.

Any idea?

Sync iBooks between your Mac and your iPhone, iPad, or iPod touch

https://support.apple.com/en-us/HT202759

Message was edited by: tiptronic (typo)