Help me plan my back workout for tomorrow (total plan overview also)

[MatthewRyan]

My goal is gain 15lbs over the next 4 months. (currently 165, 5'11") Im in pretty good shape.

Ive been working out 4 times a week, doing leg/arms/chest/back. Tomorrow is my first back day and I have no idea what back exercises would be compared in effectiveness to say.. sqauts for legs. Suggestions for my workout tomorrow would be awesome.

Also, here is my overall plan.
Diet: Im having one ready to drink Muscle Milk once a day in addition to my normal diet of 2 large meals (both with meat) ex carne asada burrito + beef taco for lunch, and 3x3 In and Out with fries for dinner. I would have the muscle milk for breakfast, 30mins or so before my workout.

Legs: The main focus will be sqauts, and after the first workout 4 sets of squats had me totally spent. Next time ill add the calf machine.

Arms: using the 1 handed grip on those rope/pully giant machines to isolate both my biceps and tricepts. Then doing skull crushers, and finishing with one arm preacher curls.

Chest: I went solo so I used dumbells to do bench. Next did the decline Hammer machine. I was spent, so im not sure what ill add next time.

I also will do some type of an ab workout each time.

Any comments would be great.

-Matt