Well, you've got 4 weeks to lose 4kg, which is 1kg/week, which is a 9,000kcal deficit per week, which averages almost 1,300kcal/day. That's not how many calories you have to eat each day, that's the size of the calorie deficit you need. That means, however many calories per day would have you maintaining weight, you need to be consuming 1,300/day less than that. So, if 2,500kcal/day would result in you maintaining your current bodyweight, you'd be consuming 1,200kcal/day (2,500 - 1,300 = 1,200).
For the record, as a 15yo with growing bones, I wouldn't recommend trying to lose that weight in that time. Rapid weight loss is a bad enough idea for adults, whose bodies aren't going to stop developing from it, because they're already developed. A severe calorie deficit for a teenager, however, can spell a premature end to puberty, which can have negative lifelong consequences.