Funny, but Julie S's approach is how I have taught my son to be aware of what is 'normal' behavior and what isn't. We kept it simple when he was four, basically reviewing Every Single Time we went to public places what to do if he couldn't find me (sit down right where you are and don't move, don't go with anyone unless it's a mommy with kids). My son is friendly with everyone, the word 'stranger' really doesn't mean much to him and it's fairly ambiguous, so we have focused on "good grown-ups" and "Bad grown ups".... good grown ups won't ask you to open a door for them (at home), to come with them, or ask you to help find their puppy, because they know ONLY Mommies or Daddies should be doing those things with them." Good grownups will ask your mom or dad before offering you a snack because that's what they are supposed to do. Likewise, if someone starts asking for help, tell them "go ask my mom". etc.
I focus on behaviors, not how well we know other people. We talk about people who make us feel good, safe and happy and other people who make us feel not-so-good, who make us want to be away from them.
And then, I consider it my responsibility to make sure he's within sight/sound of me when we are out and about. He's seven now and has moderate, manageable experiences with being independent, and we still talk about what are safe behaviors, who to go with (a mom with kids), etc.
I say all this just because the 'stranger danger' method can be harmful. One child who was missing almost wasn't found because it was an adult rescue worker calling his name, so he hid. It wasn't mom or dad, as he'd been taught, and he was just doing what he'd been told. He hid from the 'bad' stranger and almost wasn't found.
Instead, teach about what to do if someone tries to take you away: kick, bite scream, fight. We talk about how bodies can be your best weapon, too.
Keep it simple and remember, at this age, having them close when you are out at busy public places is still the best bet.
Oh, and lastly, this doesn't NEED to be an uncomfortable conversation. This is just presenting information as simply as possible; if your daughter asks about the 'bad guys', keep it simple. You want to inform without making her afraid. Remember, you have a lot of other potentially-uncomfortable conversations in the future,this is only one conversation in a long line of them about her taking good care of herself.