How to unlock the 4th barrack, I'm on base lvl 18 and still no sign to build 4th barrack. Thanks?

The fourth barracks requires base level 18 initially. There may be other requirements as well.

Go to the build menu and select 'Military' and find the barracks in the list. It may tell you what other requirement you need before you can build it. (possibly a resource, or a level of troops or population.)

Let us know if you find the answer.

There are 3 APC's and 1 class APC, have you picked a class? I'm base lvl 18 and this is the max amount you can have.