Is losing 4 lbs a week too fast?

The first week on any diet is usually a water loss week. Depending on many things such as water retention before you started, whether your diet included naturally diuretic foods like cucumbers, parsley, watermelon, etc., did you reduce carbohydrate and increased protein intake - your body weight can either drop up to 10 pounds or not at all. The South Beach Diet is most efficient for people who didn't watch their carbohydrate including sweets and white bread, who have their excess weight concentrating around the middle (apple body type) and because of that, chances are, retaining water. Keep following the diet rules, exercise as advised - and your weight loss can slow down while inches will keep shrinking and overall appearance and health will keep improving.