Guinea pigs can differ in weight considerably, but I have to admit that 4 pounds is pretty huge!
You may find this thread here helpful:
http://www.theguineapigforum.co.uk/showthread.php?t=38805
Is your boy on his own or has he company? Have you had him for long? How big is his cage and is he getting regular run time?
If he is single, a little baby boar for company can help him getting plenty of exercise, especially if you can provide regular run time.
The other area is what you are feeding him. Stick to the recommendations of
- unlimited hay (up to 80%) of the daily food intake
- limited pellets for adults - I give only 1/2 handful
-50g/1 cupful of mixed fresh veg per day. Please include at least one high vitamin C veg like a slice (bell) pepper, some broccoli or parsley. Keep off starchy food like sweetcorn and too much fruit.
- fresh water daily
- no yoghurt drops or other treats that contain dairy, sugar/honey and fats; if you want to give treats, give dried herbal mixes.