Kenny bought 4 1liter bottles of water
1liter is equal to 1,000 milliliters (milli means 1,000)
What that means is that in each bottle of water that he purchased, he has 1,000 milliliters
He has 4 bottles, so we simply take
4(bottles) x 1,000(amount of water in each bottle)
4 x 1,000 = 4,000 milliliters