From what I understand, you're doing it right. There's more benefit from two laps separated by some time than two consecutive laps.

Four miles walking is roughly 400 calories you burned, depending on how much you weigh. I'm 190 and that's close for me. Get your diet to about 1,000 calories under maintenance level, and you'll lose 2 lbs per week.

Do you have an iPhone? If so, there's an app called Lose It! that is a very effective and easy to use calorie counter. My sister lost 100# with it and has kept it off 6 years now. I still use it from time to time when my weight gets out of control. You tell it how fast you want to lose and it does all the math. As long as you don't eat crap, it works. The best part is it also tracks the calories you burn with exercise. If your normal daily allotment is 1,500 calories and you do two laps around Sloans, you get to eat 1,900 that day.

Keep it up!