It uses twice the amount of valves pictured in the diagram above. A 2-way system has 4 valves. The front bags are paired together, and the rear bags are paired together. It has two valves for each "pair" of bags. The problem with this style of stystem is it allows air to transfer between the pairs of bags. On smaller, lighter vehicles it's not a big issue, but on our trucks it makes thew sway, or lean when cornering.
A 4 way system has 8 valves total. It uses two valves to control control each bag seperately. This eliminates air transfer between bags and allows you to adjust each corner by itself.
As far as line/valve size goes, 3/8" will do what you want just fine. A lot of guys are going to 1/2" ( or bigger) but it's not really needed. If you want a system that will clown, then that's a whole different story. If you want one that gets the job done then 3/8" valves and line are plenty fast....
