In 4 weeks just running for 15 minutes a day, you may not see dramatic results but you will be burning calories.

The 'best' thing to do for more dramatic results is weight training. Or, if you don't have weights or a gym membership, pilates will work. You can see some results in 10 workouts with pilates. Although don't take the easy route and think that only working out 20 minutes three times a week will give you that benefit. You need to alternate working out with pilates or weight training with cardio (like running).

If you are trying to lose weight, you'll also need to lower your calorie consumption. Eat clean. No sugars and not too much fats. In 4 weeks you'll feel better.

Good luck. Have fun on your cruise.

You can make tremendous gains in 4 weeks, if you're training method is reasonable.

No, you will not have tremendous gains with weight training based on the assumption that you are quite new to the world of exercise. Do not attempt to do weight training without first understand the form or how it should be done. Some of my girlfriends said they have gotten bigger because they tried working out with weights, which I knew meant they weren't doing it properly. 15 minutes a day for the next 4 weeks... I do not think that is adequate. Pilates is great, but its primary focus is core strengthening. I would personally recommend to you a combination of circuit training/cardiovascular work outs, but 4 weeks is not enough time I'm afraid for a new starter to lose weight drastically.

You say you are 137 pounds, but how tall are you? What kind of a life style do you have and do you do any physical activities regularly. As Ingi said, you would need to decrease your calorie consumption but we do not know what and how you eat either to give you straight answers. Do not decide to stop eating for the next 4 weeks however, you may simply need to modify it.