you should be looking at the combined protein of foods as well as protein powder throughout the day. If you are looking to bulk up, at your weight you are approx 210lbs, so you should be aiming for about 250 - 300 grams of protein per day.
So given you have 4.5 scoops a day, thats approx 130 grams of protein. So you would/should have to make up the rest of the requirement from food.
It is common thinking that you treat powders as a 'supplement' only and that real foods are better protein sources, I would say that 4.5 scoops a day is about right, anything above 6 scoops and really your relying too much on powdered protein sources over food sources.
a scoop or two before hitting the hay - YES - milk will slow down the protein digestion.
cassein protein -
http://www.myprotein.co.uk/bulk-powd...ium-caseinate/
cassein is better than whey in milk but i dont think its really all that important.