A quick problem for tutors and learners

You find yourself in a weird situation (let's leave it at that).  You have to measure out exact amounts of petrol to travelers in a post-apocalyptic NZ environment.

You are going to tip one litre of petrol into the first car, two in the second, and three into the third car and so on.  This will continue until car number nine who will get nine liters.

The problem is that the petrol comes out of a hose attached to a large tank, and you have no measurement devices.

What you do have is a four litre and a nine litre container.

How do you measure out exact amounts using only these two containers?

Here is a start: Measuring eight litres

Fill the four litre and tip it into the 9 litre.  Do this again.  You now have eight litres in the nine litre container.

That's eight done. Can you work out how to measure all the others?