If you're high and you have nothing on you, then there is really no way you can get in trouble. Not unless you admit to being high, even then I don't think they can do anything about it. Just don't be stupid and admit to it, just in case. You aren't affecting anyone elses life by being high in public, so if someone has a problem with it they can go to hell. Just tell them you support the legalization and maybe you can educate them. Besides, anyone that would have a problem with you being high on 4/20 is probably ignorant to the truth about cannabis.