How quickly have you lost the weight? Are you increasing your calories enough to compensate for the energy you expend on your rides?

You need to make sure you are taking in enough calories to fuel your ride. I looked up the 4 hour diet, and without spending a lot of time looking at the details, it doesn't seem too outlandish, as many "fad" diets do- as long as you get a good mix of the recommended foods.

But if you aren't getting enough calories (including some from carbs), you will bonk. Nancy Clark's The Cyclist's Food Guide will give you some good guidelines as to how much you need to eat. Food is your energy source, and just like you can't drive your car without gas, you can't ride without enough energy from your food. It is hard to lose a significant amount of weight and keep up your energy for cycling. You might need to slow down your weight loss.