In the equation \( y = 4z \), the constant of proportionality is the coefficient of \( z \), which is \( 4 \).
This means that for every unit increase in \( z \), \( y \) increases by \( 4 \) times that amount. In other words, \( y \) is directly proportional to \( z \) with a constant proportionality of \( 4 \).