Hi, welcome to the forum. I have to say that your beardie should have grown quite a bit in 4 months so something definitely isn't right and needs changing.
You mentioned that you're using a Repti-Glo 26 watt UVB, is it a strip fluorescent or coil/compact bulb? Are you using a bulb that produces heat during the day? If so, what is the wattage of it and is it a colored or white bulb? A 100 watt bulb at night is way to much wattage for only a 15 gallon tank. They need it much cooler at night so if your room temp doesn't drop lower that 65 degrees, you don't need any heat at night.
He isn't getting enough to eat, at his small size, he should be getting all the crickets he can eat in about a 10 to 15 min timeframe 2 to 3 times a day. Did the pet store tell you to only feed him 3 to 5 crickets a day? That is nowhere near enough, he needs lots of protein to help him grow. Feeding carrots once in a while is ok as long as they are fed with healthy greens, he needs to get dandelion, escarole, endive, mustard greens, turnip greens, collards, etc. Carrots are high in Vitamin A so they shouldn't be fed daily by themselves. Here's a site that shows the best to feed in green ink:
http://www.beautifuldragons.503xtreme.com/Nutrition.html Are you dusting his crickets with calcium & vitamins, if so, how often? Also make sure the crickets are no larger than the space between his eyes or he could choke or get impacted.
Babies can get dehydrated very quickly so if you haven't been
bathing him, it would be a good idea to give him a
bath in warm water tested by the inside of your wrist, it shouldn't feel cold or hot. A 15 to 20 min soak is very beneficial but be sure to watch him closely as they can drown in a small amount of water. There should be enough water to cover his front legs and be sure to put a washcloth down in the bottom to help him feel more secure.
What type of thermometer are you using to check the temps and what are the temps during the day?
Please fill us in on the unanswered questions and let us know if anything needs to be clarified.