You may have already discovered this, but your professor is referencing Francisco Bulnes' El porvenir de las nations Hispano-Americanas (The Future ofthe Hispanic-American Nations), which divided the world into 3 "grand races" (wheat, maize, and rice eaters) and then asked the very simple question, "which of these are indisputably superior?" Bulnes' conclusion is that wheat eaters were by far superior due to some long-winded handwaving about nutritional value, and thus European domination was "explained."

Bulnes did not classify a fourth race though, unless I'm missing something. Like with many questions about something a professor said in class, this is probably something you should ask him about.

Perhaps he's thinking of root vegetables, like potatoes... They're a staple in many countries. I would recommend you email your professor or ask him after class!

Possibly millet, which is a major cereal grain which appears to have first been domesticated in Asia and wended its way to Europe and Africa. Millet isn't usually the sole grain cultivated in ancient cultures, and in some of them at least it was preferred as animal fodder and/or millet consumption was largely relegated to lower social classes, but in the more arid parts of Africa and Asia it was a staple food. There's a decent overview on the subject in Adam Scott's "The History of Millet".

I'm not sure there are four. Rice, wheat, and corn combine for something around 60% of all calories consumed worldwide (according to the UN Food and Agricultural Organization). There's nothing else that really matches those big three.

There are a lot of grains and roots that are local staples: millet, sorghum, potatoes, cassava, yams, taro, rye, soybeans, barley, oats and teff being the standouts. Of those cassava is probably the next biggest one. And if you were to combine it with potatoes, yams, and taro they'd probably be a decent fourth "roots and tubers." But that seems a bit like cheating.