Hate to break it to you guys, but the range of motion is only half the battle. The perfect form to maximize the stress on the Pecs only, involves more than stopping 4''from the chest or not. First off, your grip should be wide. I know many guys in the gym that have complained to me about hurt triceps, or bum shoulders and it's because their packing the weight with a narrow grip placing much stress in the shoulder first, and the triceps second with minimal stress on the pecs. These are also the guys with the smallest chest. so keep your grip wider than shoulder width.
Second, keep your shoulder blades together on the bench, with an arch in your lower back, and feet flat on the floor, knees out comfortably. Don't make like a lat spread when you're on the bench, or you'll put the stress on the frontal delts.
Lastly, don't roll your wrists backward on the pressing motion. Keep 'em tight, and try to keep your knuckles almost facing the ceiling. I like to keep my thumbs out, but that's up to you.
As far as letting the bar touch your chest or not....do whatever u feel doesn't bother any joints,or tendons and pay close attention as to where u feel the pump after your set. If u don't feel it mostly in your chest, then make an adjustment on where u r stopping the bar at the bottom of the motion. With these rules of thumb, I feel about 90 percent of the stress, and pump in my pecs, and very little if any pump in the shoulders or arms. My chest is very tight, vascular, and well proportioned.