Again, I just don't like the term "secondary class" being thrown around at all. I think this is confusing people by oversimplifying things. I'd prefer the concept of secondary abilities instead, as this is how it has been described by the developers. This is different from just having an interchangeable set of secondary abilities which are inherited wholesale from different classes in groups of 4.
To make very clear what I mean when I say your secondary abilities #5-8 are not just from one class, let's use my previous hypothetical example of the mage's secondary skill template: 2 mobility skills, 1 damage skill, and 1 pet skill. In this case, you could be playing that mage with 1 rogue mobility skill, 1 warrior mobility skill, 1 necromancer damage skill, and 1 ranger pet skill. This build would require you to have those classes unlocked, of course.
Any class you unlock has some secondary abilities which fall into specific categories. Different primary classes have different templates for what secondary abilities can be "plugged in." (You'll note that my example of the mage does not have healing or damage reducing abilities available, but other primary classes would.) As you unlock more classes, the ability to customize builds will increase quite a lot.