When a coin is tossed once, there are two possible outcomes: a head (H) and a tail (T).
When a coin is tossed four times, the total number of possible outcomes is 24 = 16.
Thus, when a coin is tossed four times, the sample space is given by
S = {HHHH, HHHT, HHTH, HHTT, HTHH, HTHT, HTTH, HTTT, THHH, THHT, THTH, THTT, TTHH, TTHT, TTTH, TTTT}