The leading coefficient of a polynomial function is the coefficient of the term with the highest power of the variable. In the given quadratic function f(x) = 2x² + 5x + 4, the highest power of x is 2, which comes from the term 2x².
Thus, the leading coefficient is the coefficient of this term, which is 2. Therefore, we can say that the leading coefficient of the function f(x) is 2.