Trying to keep this from getting too pedantic. You'll rightly lose interest quickly. I'll keep trying.
Fourth graders aren't too young to be taught how to recognize the truth of anything (or the untruth of falsehoods). I found myself doing this naturally as a youngster when a teacher told the classroom that the country was named Yugoslavia because its inhabitants were slaves. I remember this word ringing in my head: "Really?".
Questioning the message -- not the same as defying authority -- is a great habit for young people to develop. It starts with these questions: who created the message and what creative techniques were used to attract their attention? You can easily pick your own example. I'll start with something easy: a perfume ad in a magazine. Magazines (or catalogs) might enclose a sample scent sticky-printed onto a fold-over. Unfold it and you can sniff to get an idea of what the perfume smells like. Creative! Shiny, sparkly crystal atomizers on a dark background. Creative! A sunken-cheek, liquid-ruby-lipped (pouty, of course) female model near the perfume bottle. Sexy! Er, I mean: creative!
Then our young people should be taught to contemplate how different people might understand this message differently. Not the best example to follow, so let's limit it to how men might interpret it differently from women. Men might see the model and understand that females probably prefer this perfume. A woman might understand that if she uses that particular perfume, she will instantly become as glamorous as the model in the ad.
Also important to consider with any message are the values, lifestyles, or points of view represented in (or even omitted from) the message. Well...you understand where I'm going with this. You can apply your own values, interpretations of lifestyles, and points of view to the perfume-ad example.
And, of course, the young people should consider (ask) why the message is being sent. Too easy in this example: somebody is trying to sell a product.