Measure a milk without exact scale

This is quite easy. First you fill the 5l jug, then pour out of it as much water as needed to fill the 3l one. When you finish doing that you'll have 2l left in the 5l jug. Step 2: you empty the 3l jug and then fill it with the 2l you had in the other jug. Step 3: Fill the 5l jug and then pour in the 3l one as much water needed to fill it to the brim. Because you had 2l in the 3l jug you now poured an extra liter, meaning you now have 4l in the 5l jug :)