Am I losing weight too fast?

I'm on the Special K diet right now, but I changed it a little. I'm eating a bowl of Special K cereal for breakfast, some random brand, 230 calorie protein shake for lunch (I couldn't find the Special K in the store), Special K bars for snack twice a day, and usually some sort of meat and vegetable for dinner, just normal dinner food. I'm also drinking green tea twice a day and I go from hour-long walks almost every day. I'm 5'8'' and I started at about 150 pounds and I want to reach 135. I'm a 15 year old girl. I used to be very skinny, but gained weight from eating when I wasn't hungry and lack of exercise I started this diet on Monday (6/27) and I am now 146. I'm glad that I seemed to have lost weight, but I'm a little concerned about the rate. 4 lbs seems too fast right? Does this seem bad or strange to anyone else? Is there a reason that I'm losing so fast?