You are 18, and it may grow. It may not. 4 inches is small, but not unusually small. Average is purported in some studies to be about 5 inches. All studies are suspect though, and some put the average closer to 6 inches. Try not to let size disrrupt you life, whether it remains 4 inches, or somehow explodes to 9. Either way your life nor your world should revolve around your penis. So, go on. Ask people out on dates. Pursue your career. Plant a garden. Paint a landscape. Or lay in bed and smoke weed. Whatever.