I am designing one or more garage storage cabinets. They will be 4'x8'x16" and the carcass and shelves will be made from 3/4" MDO plywood. Question: would a 1/2" plywood back with a series of shelf pin holes down the center help with shelf sag?

The pins would help. Facing the front of the shelves with a 1X1-1/2 would really help. 

You didn't say if the 8' was horizontal or vertical. If horizontal, a couple equally spaced stiles with pin holes would be excellent. Especially, if your going to install doors.

It will, just as Gene

suggested I would also face the front of the shelves with a stiffener. I built a huge bookcase with 48"x 11" shelves and did exactly that, and there was no sag at all with the books my wife piled onto them.

Edited January 7, 20179 yr by Fred W. Hargis, Jr