Its just 4 sticks that are pretested & verified by manufacturer to run in quad channel together.
If you buy 2 more additional sticks of the EXACT brand/model that you already have, they may all work together nicely in quad channel. I would say odds are in your favor, but you may end up stuck running them in dual channel. But even if that happens, doubt youll notice the difference.

Mixing brands or even models will definitely not put the odds in your favor.