I am probably overthinking this but this is the kind of thing that my brain does to me.
First, here is why I think the original question is flawed. None of the four ingredients of beer is more important than any of the others. If you take away any of them, you cannot make beer. Hops is the only one that you can remove and still brew a fermented beverage but, according to the modern meaning of the word beer, without hops it is something else.
Now, the point he was trying to make – and it is a good one – is that yeast is has a greater effect on the nature of the final product. There are thousands of species of yeast in the world and hundreds that are used to ferment beer. Each species acts a little different from the others. Some are incredibly efficient at eating sugar, converting more of the sweet stuff into alcohol and CO2 than the others. Lager yeasts, for example, tend to be better than ale yeasts at eating sugar. This means that lagers come out of the fermentation tank drier and with more alcohol than they would have if an ale yeast had been used.
Some leave behind flavors that are essential the style. A good example of this is the ale yeast that is used to make hefeweizen. The banana and cloves aroma and flavors that make this style so distinguishable are a direct result of the yeast. The small bit of yeast that collects in the bottom of the bottle serves to enhance these flavors when it is mixed in with the beer.
Other yeasts are used specifically because they are less efficient fermentors. Beer styles that tend to be sweeter and retain more of the flavors from the barley are best fermented with ale yeast strains that do not gobble up all the fermentable sugars in the wort.
So, when considered from this perspective, yeast is the most important ingredient for determining the flavor, quality and style of beer. However, without the other three, you simply could not make beer.
Like I said before, I'm clearly over thinking this. But, it is fun to consider these kinds of things. Beer is a pretty incredible thing and it is worth the time to wonder about how all these ingredients come together to form this drink that we all love.