When the 4-minute one runs out, turn it over immediately so it starts counting 4-minutes again
When the 7-minute one runs out, turn it over so it starts counting again
The moment the 4-minute one runs out for the second time, turn the 7-minute hourglass over - it will have only been running exactly one minute.
Let the sand run back again (1 minute more) and then straight away, 9 minutes.
(4 minutes twice, plus one more minute = 9 minutes!)