Why A single flip-flop configured as a toggle changes state once per input cycle transition, so its output is half the input frequency. Chaining them multiplies the division: n flip-flops give a divide ratio of 2^n. Since 2^4 = 16, four stages are needed.
Watch out Picking 16 assumes one flip-flop per count, but the division is binary and exponential, not one-to-one. Eight flip-flops would divide by 2^8 = 256.
Divide ratio = 2^n. Count the powers of 2: 2, 4, 8, 16 is four flip-flops.
Sweet sixteen is four doublings: 2 x 2 x 2 x 2. Four 2s, four flip-flops.