The feeding of the 4,000 might seems like just an editorial, as if St. Matthew was just repeating the same story in slightly different form. But the deeper point is that Jesus doesn't run out of miracles. There's always more where that came from. More bread, more grace, more mercy. We don't have to be afraid that God is going to run out.
Jesus then warns his disciples about the "yeast" of the Pharisees and Sadducees. The disciples think that he is talking about real bread because the forgot to pack lunch (again!) and seem doubtful that Jesus can provide a meal (again!). But Jesus is instead speaking about their teachings as yeast. "Be careful what you mix into the bread," Jesus says, "As the yeast is so is the bread." When we mix legalism, demand for signs, doubts about the resurrection into our faith, we should not be surprised if the loaf of faith does not rise.