Hi Valority,
At 15, the gym 4 times a week and pure calories with plenty of protein will cause all kinds of gains.
Just so you know what is in most mass gaining powders, its effectively a bunch of sugar and carbohydrates in a milk powder. There's no magic stuff in there, just dextrose. in fact, in general it basically just mimics milk in nutritional profile, without the other good stuff in milk. Usually useful if you can't keep milk down.
A lot of people will recommend being less aggressive with carbohydrates and sugars, but they're not 15 anymore and tend to forget what having that level of testosterone is like.
I will say this though, before you move to any sort of supplements, make sure you're eating 3 square meals a day with plenty of protein and calories from whole foods as you can get.
At 15, if you're regularly strength training, you should be looking to eat over 3000 calories a day, at least, but it's hard to say without knowing how heavy you are now, or what kind of gym work you're doing 4 days a week.
You'll put on a bit of fat on any sort of effective mass gaining program/diet, so don't panic if you lose your abs for a little while.
When you've used up the mass gainer powder (try to take it after workouts), if it's cheaper to swap to a litre or more of milk, I would definitely recommend that. it's usually cheaper, and contains actual nutrition that is of benefit to your general health, fats, calories, proteins, links to IGF-1 and calcium... it's the magic bulking juice.
To qualify what I'm saying, I'm a Personal Trainer with most of my experience in getting people bigger and stronger and more effective for combat sports.