Meh, I've had upwards of 1g of pure caffeine (tastes really bitter btw, disgusting stuff...) in one setting, comparable to 7-ish cups, and noticed no ill effects. It really does differ from person to person though. By itself, caffeine is harmless up to 3-ish grams over 24 hours unless you have some sort of heart condition. The main actions of caffeine are as an adenosine inhibitor (which will make you feel awake, but will result in a "crash" after high doses). This also has the very noticeable side effect of twitchiness and irregular heart beat at high doses. Contrary to popular opinion, caffeine doesn't cause tachycardia (fast hear beat). One fun use of it is before exercise, as it will increase your endurance by a fair bit.
Anyway, enough rambling about my drug of choice. 4 cups of coffee are approximately 600mg of caffeine, which is not dangerous unless you have a heart condition. The only ill effects you might notice are in your digestive tract, as coffee isn't all that great for you. As far as addiction is concerned, there really isn't any. You might feel crappy for a day or two after quitting cold turkey, but that's it. You will build up tolerance though, so moderation might be advisable.