OK, so here's my explanation for what it's worth:
Set both timers
When the 4 minute one has finished, turn it over and restart it;
When the 7 minute one has finished, turn it over (there is now 1 minute left in the 4 minute one);
When the 4 minute one is empty again, turn the 7 minute one over quickly - it has only filtered one minute's worth of sand through to the bottom, so when this has gone back through, you'll have had 2 x 4 minutes worth and that 1 minute from the 7 minute one, making 9 minutes in total.
Phew, that looks ever so confusing Miss, sorry. I hope it makes sense