Using negative numbers to represent weight loss and positive numbers to represent weight gain, the change in weight over the four weeks was as follows:
Week 1: -2 pounds.
Week 2: -7 pounds.
Week 3: +1 pound.
Week 4: -4 pounds.
Add all the numbers to find the total change in weight over the four week period:

Therefore, the change in their weight over the four weeks was -12 pounds.