Looking at the diagram below, at the start on the left, ABCD are all high, 1111, equivalent to decimal 15.

After clock pulse 1, A has gone low giving 1110, the equivalent of decimal 14.

After clock pulse 2, the count is 1101, decimal 13.

After 8 pulses the count is  0111, decimal 7.      15-8 = 7

After 15 pulses the count will be 0000.

On the next pulse the counter will indicate 1111.