They are chipping because something has changed - so most likely causes are: 1. They haven't made the same bite as you have before when you chew or grind; 2. You are grinding more because you aren't comfortable with the changes with your bite; 3. Something is faulty with the porcelain (least likely); 4. Some other unforeseen change in your bite?? Hope this helps.
Different types of whitening are primarily time versus concentration in how they work. Grey teeth is usually increased translucence though sometimes it might be exposure to metals, particularly tin. While I haven't read any studies, my personal experience is that Zoom is a bit better than tray whitening with gray teeth - likely the intensity changes the translucence more. My advice is to do it better you do the crowns. Good luck!
The best material depends on the individual case - CEREC is a process, not a material. Unless you use gold, it gives the dentist a choice of medium strength materials. Gold is the best for wear and won't fracture. For least tooth material reduced, good strength and low wear, CEREC using eMax would be my choice. But again, choose a dentist that you know has all the materials and techniques in his skill set.
Fast and cheap gap closing in done using dental bonding. You might benefit from a little bit of careful reshaping of the gum in the midline as well. Expect the bonding to last 3-7 years, with the main problems being staining or chipping - both of which can be dealt with by your dentist without replacing the bonding.
The implants themselves will not - the setting of the teeth and bite can make some difference. This is also possible with a denture. For the cost of the All on 4, I would suggest making a new denture with the teeth and bite in the location planned for your All-on-4 prosthesis - then you will know and your dentist will be sure heéshe is doing what you want.
