If i lose 4lbs a week through dieting alone, would it be mostly fat, muscle or water loss. not intending to lose that much but very curious?

It depends: The content of the diet, calories consumed, and even levels of exercise/activity, can effect if primarily lean tissue or fat is consumed. With most dieting the first weight loss is often attributed to fat stored in the liver being burned off. There is clearly controversy about the safety of popular diets..ideally patients can afford individualized care by a qualified professional.