I had one of these read to me and it just scared/scarred me. I dont think books are necessary. Just make it clear that you will always let DC know if anyone other than you or Dad is going to pick him/her up from somewhere, and they are not to go with anyone else even if its someone they know and trust, unless you have told them its OK.
Yes, especially as child abductions are frequently by someone the child does actually know - a friend of the family even. Just stress that your 4yo never gets into a car with someone other than you unless you've told them that someone else will pick them up.