A chef has a recipe that requires exactly 4 liters of milk and offers you $20 for it. You only have two similar cylindrical cans (one holds exactly 3 liters and the other holds exactly 5 liters). Your source of milk is metered and you are charged $2 for every liter dispensed. Conservationist tax will charge you $1 for each liter dispensed that not used in the recipe. With just the cans for measuring, what is the most profit you can make?

The cans are unmarked and start empty. Inaccuracies caused by minor things such as the thickness of the cans, loss of milk during multiple pourings, the ability to pour right to the rim, surface tension rise above the rim, etc. are negligible.

Note: There are multiple solutions that can get the 4 liters but I know of only one that turns a profit.