An interesting analogy from Rabbi T:

Participant: Theoretically, three legs should provide enough stability for us to sit. So why do we usually sit on chairs with four legs? The three patriarchs represent the three pillars that the Jewish nation was formed on. Abraham represented hospitality and kindness, generously hosting and housing even strangers. Isaac is more introspective and devoted. Jacob is a mix of the two.

The four matriarchs though, are rooted in reality. They translated their husbands' theories into real world actions. For example, Sarah fixed the dispute between Ishmael and Isaac, while Abraham just told them to figure it out themselves. So even though explicitly reference the patriarchs, the four matriarchs are remembered throughout the Seder through the number four. We drink four cups of wine. We ask four questions. And there are four sons.

haggadah Section: -- Four Questions