Before all four flips, the chances of getting 4 heads in a row was 0.5 x 0.5 x 0.5 x 0.5 = 1 in 16
After 3 flips have come up heads already, the chance of the FOURTH ONE coming up heads is 50/50
but you can't neglect the other three flips because without them you wouldn't have the chance to get a 4th head in a row. So 1/16 is the correct answer








