How to teach stranger danger?

[marchduck]

This has been on my mind for a while, is there any way to teach it in very basic terms?
DD is 4.3, I am trying to work on her responding to simple questions; sometimes she will, more often she won't!
She has no concept of the idea of strangers, let alone stranger danger. I am flummoxed about how to try to introduce this subject, given that I've been trying to encourage her to engage with other people - totally mixed message?
She is always supervised at home and at her brilliant pre-school. I'm conscious though that she will be starting primary in September and whilst school has locked door etc; I feel less in control, if that makes sense. Any advice very much appreciated

Thanks zzzzz, and I get what you are saying about focusing on the intention causing the worry, that makes sense.
i guess what I'm really worried about is her getting lost; without having the capacity to tell anyone that she's lost. I had bought her an engraved dog-tag bracelet with my contact details to wear, in her favourite colour, but she won't wear it.

zzzzz, that's a great idea about the mobile number written on hand; I will be doing that - thank you