4 weeks and lose about 1 stone. It can be done, I managed 7kg in 1 month. However, that is through some good cardio, workout sessions and diet.
1) I did my cardio in a way that it keeps your heart rate somewhere between 130-150 bpm
2) I did some workouts to build some muscle mass, but I guess swimming is a good substitute.
3) For diet, all I ate was a can of tuna and vegetables a day.... and protein shakes to maintain my muscle mass and curb hunger.