Once the above overs have passed, you can not achieve a bonus point by hitting a 4 or 6 off the final delivery!
"Whenever a target, or revised target, is set, the exact number of overs/balls within which the side batting second must achieve this target in order to gain the bonus point should be announced. Once these overs/balls have been bowled, if the target has not then been achieved the bonus point cannot be gained by any subsequent event, e.g. a multiple scoring shot, or extras."