1. Start both hourglasses at the same time.
2. When the 4-minute hourglass runs out, flip it immediately.
3. When the 7-minute hourglass runs out, flip it immediately.
4. When the 4-minute hourglass runs out again (at 8 minutes), there is 1 minute of sand left in the 7-minute hourglass.
5. Let that 1 minute pass. You're now measured exactly 9 minutes.