Fill the 9 l and pour it into 4l , pour out the 4 liter
do it again, pour it out
that leaves 1 liter in the 9 liter
pour it into the 4 liter
fill the 9 liter and pour it into the 4 liter to the top, 3 liters will fill it
leaving 6 liters in the 9 liter container