Measure 4 liters with a 7 liter and 5 liter?

Fill the 7 litre and pour into the 5. there's now 2L in the 7L jug. Empty the 5L and pour the 2L from the 7L into the 5L. Refill the 7L and top up the 5L. It will take 3L to do this, leaving 4L in the 7L jug.