Exercising 4+hours a day is a
recipe for injury and burnout. Is it something that could be built up to? Of course. Take a look at professional cyclists, some of whom train 30 or more hours a week. Is it something that someone should start out doing? I wouldn't recommend, and nobody I know would either. It's a
recipe for disaster.
Most of the stuff you will read says that healthy weightloss should fall around 2-3
pounds a week, or around 1-1.5 kg's. Being overweight is not healthy, but neither is losing huge amounts of weight too quickly. As mentioned by previous posters, proper
nutrition and the gradual increase of exercise is what you need.
Keep a log of the
calories you are eating on a daily basis, it will give you a good idea of why you may or may not be losing weight. Be careful that you don't go into "starvation mode." Eat balance, healthy, smaller portioned meals 5 or so times a day. Without knowing a lot more information about you, I wouldn't be able to recommend around where you should be on a daily caloric intake basis.
If your doctor has given you the okay to exercise, I would recommend you begin your exercise program with a low intensity, and slowly work on increasing the amount of time you work out. There are three factors in all workout programs I design - volume (amount you're working out) intensity (how hard you're working out) and rest/recovery.
Someone just starting out should develop a good aerobic and strength base, then begin increasing volume while keeping intensity lower. Later, you can begin increasing intensity. The harder and longer you go, the more rest and recovery your
body will need.
Stick with it, and best of luck.