At 4 months, although that is a major achievement, your quit is still in it's infancy. As Jillar pointed out, things will keep getting better and better as time passes. Your battle now is a mental one, focusing on the benefits you've realized since quitting and seeing the lies that you told yourself as a nicotine addict when you smoked. Back then it was your addiction talking and now YOU are in charge, giving that addition thinking the boot.
Seriously, the more you realize the benefits of quitting vs the lies of addiction the easier it will be to remain smoke free forever while actually enjoying it!