Let’s call the 7 minute hourglass A and the 4 minute hourglass B.

Start the 2 hourglasses simultaneously.

When B has been emptied, flip it. 4 minutes have passed since the start.

When A has been emptied, flip it. 7 minutes have passed.

When B has been emptied again, we’re at the 8 minute mark, and 1 minute worth of sand should have flowed down in A. Flip A.

When A is emptied, we are at 9 minutes.