The penis is not a muscle
Contrary to popular belief, the so-called love muscle doesn't contain any muscles. That's why you can't move it very much when it's erect. The penis is a kind of sponge that fills with blood when a man is sexually excited. Blood builds up inside two cylinder-shaped chambers, causing the penis to swell and stiffen. The swelling blocks off the veins that normally take blood away from the penis. As an erection disappears, the arteries in the two chambers narrow again, allowing blood to drain away from the penis.
- Source: http://www.nhs.uk/Livewell/penis-health/Pages/Five-penis-facts.aspx
4.9" erect is OK for a 14-year old. Development takes place at different rates at different ages and there is nothing to do to speed up the process. Why do you want to? The average adult erect penis is not much bigger at about 5.5".