I am by no means a specialist, simply a girl trying to change the world one smokey eye at a time. Before I had filled my quad, I always wondered what shadows to choose — I think I have it down now. If you’re going for only 2 colors, stick to having a transition color, and the perfect lid shade.
However, if you really wanted to fill all 4 spots in your quad, I suggest: a warm/cool transition shade — I use Red Brick or Brown Script, a the perfect all over shade — I use Mulch, a shimmery lid shade that could also work as a brown or inner corner highlight — such as Expensive Pink, and a dark shade to add some dimension such as Sketch.
These are obviously going to change based on your preferences, but I feel that this could for work for most people.