Displaying an image if in particular category

Serious newbie here - I’m trying to work out how to display an image if the category the page falls into is either 3 or 4.
I’ve tried something like this, which didn’t work.

[php]

[/php]

Advanced apologies for how simple this no-doubt is…

you’re close
[php]
if($cat_id == 4) {
echo ‘’;
}
[/php]

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