When a coin is tossed once there are two possible outcomes : Head (H) and 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}