could you help giving me explanations on this problem ? Thank you.

Let X be an unknown random variable. We ran n experiments for X, we denote by Y; (i > 0) the number of samples that are equal to i. Suppose that X is a binomial random variable b(4, p) with p unknown. the maximum likelihood estimate p is equal to Y1 + 2Y2 + 3Y3 + 4YA D = 4n b) Show that the formula for p in part (a) is a sufficient statistic for p. (Give a rigorous proof. )