6'2" and 15 stone 12. lost 4lbs in 1 week with very little exercise and eating 1700 calories a day. is this normal?

It depends : Your weight can fluctuate by 10 lbs a day. 4 pounds may just be water weight. I believe a man your height should be eating 1800 calories per day. TO BE HEALTHY, YOU SHOULD NOT LOSE MORE THAN 2LBS PER WEEK. IF YOUR WEIGHT CONTINUES TO DROP WITHOUT TRYING , YOU SHOULD GET A MEDICAL EVALUATION AND BASELINE LABS TO CHECK YOUR THYROID, LIVER FUNCTION, AND INFECTION.