Thats good (gomad) if you are an ectomorph but could be fattening if you are not. I know this from my own experience but the additional amount of calories will always result in gaining some fat. You have to know that gomad/shitloads of milk is casein in 80% and it does not necessarily build muscles in the way whey does. Casein is an anticatabolic protein (means that purpose is to prevent protein degradation primarily) with extremely SLOW absorption rate (low muscle building potential) while whey builds muscles in the first place and involves really fast intake rate. Its not that cheap if you think about it for a second, salmon bellies are lots more calloric (100g is 370 kcal) and contain lots of whey proteins of highest value, not to mention about healthy fatty acids omega 3. Another thing is that drinking 4 liters is unpleasant in comparison to 500 grams of salmon daily.
Additionally, lots of people are lactose intolerant, and lactose is not a good carbs source AT ALL.