In US they sell milk in different containers, one of them is in a gallon.

one gallon is like you said, 4L.

Back when I was there 1 gallon is 4 usd, which is about rm 16. however to get 1 gallon here you probably need to spend near rm 30.

if you have the money for it, I recommend that you don't go this route to bulk. instead eat nasi kandar twice a day which should be rm 20 to 30 depending on where or what you eat. this will make you feel full with mix of protein, carbs and fat along with some fiber on the side if you go for veggies.