How to talk to my son about strangers?

My son will be four in a few months and he's very friendly and social with everybody all ages. I've always been worried about this, but recently there was a scary situation of a stranger parked late at night down our street when I got home. I didn't want to leave my truck because it was just my son and me. He was anxious to get out. I told him we couldn't because there was a scary man outside. This calmed him down, but I think it made him more curious than scared. I wanted to talk to him about strangers last night, but didn't want to put that thought in his head before he went to sleep because I didn't want him to have nightmares about it. I know I have to have that talk with him, but I don't know how to make him understand. He will be starting school this fall and I know they talk about that stuff in school, but given the recent situation I don't think I should wait any longer.