It all depends how it is plumbed, this would help determine if its an advantage. The three line set up sounds like a "factory set up". One line for the rear and one to each front. Depending how it is plumbed, the four line would not really be any different except they maybe trying to achieve more rear brake with individual lines to the rear. Knowing how it is plumbed would be the determining factor in why. The square block you are seeing is used primarily because they are using reservoir bottles with only one outlet. The master has two inlets, the block adapts the one line to feed two inlets. No advantage.