Draw the first line beginning at dot 1 and continuing diagonally down and right, pass through dot 5 to reach dot 9.
From dot 9, the second line is drawn upwards, passing through dot 6 and dot 3 to end above dot 3.
The key to solving this puzzle with 4 straight lines joined continuously -- often described as "drawn without lifting the pencil off the paper" -- is to end the second line far enough above dot 3 that the third line drawn diagonally down and left will pass through dots 2 and 4, ending left of and level with dot 7.
Finally, the fourth and last line is drawn horizontally across to the right, passing through dots 7 and 8, ending back at dot 9.