Surprise, surprise, surprise! Low carb or even just low sugar does it's magic yet one more time! Of course 9 lbs in 4 days is a bit much even for low carb as far as fat loss goes, but it is well known that a bunch of that initial loss is water. But even if you lost 1 lb of fat in 4 days that would be spectacular. Just don't be surprised if you get a bump upwards as the water somewhat equalizes, all has nothing to do with regaining fat. Once I removed 42 lbs in 6 months with low carb(and never hungry) and it was always an up and down as far as the daily weighing, but still the overall trend was down with lower highs and lower lows. But, no matter what the scales were saying, my gut just shrunk continuously. And all things health related(blood work, blood pressure, heart burn) immediately and continually improved.
And regardless whether you have lost more fat vs water right now, your report about
"joint pain diminished sharply, endurance, vitality and strength increased and I'm no longer tired" speaks for itself! Good for you, keep it up! In fact, I have to get back on that wagon, I have been putting it off.
One tip in case you have not already heard it: if you find yourself with the blahs while trying to eat this way, keep in mind that having that much water leaving so quickly, it tends to drag salt with it, and some other minerals also, until it stabilizes after a week or two. If you start feeling bad, and especially if you find your blood pressure dropping quite a bit, add some more salt to your diet. According to the authors of "The Art and Science of Low Carbohydrate Living", Jeff Volek PhD, RD and Stephen Phinney MD, this is a common problem during the first 2 weeks.