As someone who owns a small cabinet shop, our defaults/quality standards are 3/4" carcass with 1/2" backs and solid 3/4" wood face frames. That said, many people out there use 1/4" backs, 1/2" carcass, edge tape, and so on. If you build the boxes individually and lock them together, in reality the sides will be 1" thick between cabinets. The two main issues would be strong joints between the backs and sides, and face frames and sides, and the shelving. I'd strongly caution you against using 1/2" for your shelving unless you put a strong wood nosing on it to prevent bowing. If you're going with a 4' box or wider, I'd suggest 1 1/4"-1 1/2" solid nosing on the front and back (I'd do this with my 3/4" stock at such a wide span). Alternatively, put one or two partitions in there to shorten the shelf length (you're going to need at least 3 doors at that width cabinet, so you can line up the partition with the odd door)
I hope this is helpful.