Every boxer grows at different rates than others. What kind of food are you feeding her? Food quality has a lot to do with growth. Also, you can treat with peanut butter filled Kongs, add eggs to the food, give a peanut butter sandwich, add yogurt to the food, etc.
I was in your same shoes when my girl was a puppy, even now people still ask how old she is (she's two so for the most part, done growing!). Sometimes you just get a peanut! Just like every human is different so is every boxer (although there are standards they should meet, so they should therefor be within a specific weight range)! My Bella tips the scales on a good day at 38 pounds (she's not only very short, but very very thin).
Yup, Derrick got it right.. Look at the parents and grandparents. A lot of it is genetics, but occasionally you will just have a smallish Boxer..Heck look at Kaci, she's a great example, and I know we have a couple others as well....It doesn't mean there is anything wrong...Have you asked your vet? Boxers like any dog, grow at different rates, so if yours is healthy, I wouldn't really be concerned. You cannot MAKE them grow...
Or maybe the four month old is too big - not everyone breeds with the standard in mind and to a lot of people, "bigger is better" so there are some Boxers that are far out of the "medium-sized breed" definition.