That's a pretty big pay gap to be explained by discrimination. It would be a lot easier to believe if 10 people were getting $4K less each.
There are good reasons to pay people different amounts for doing the exact same job. Someone with a broader skill set that can fill other positions if needed is worth more, even if they never actually need to do that other work. Sometimes having a degree in the field gives legal cover if anything goes wrong, even when a lower-paid person is doing the same work just as competently.
Admittedly it's hard to see either of those applying to a marketing role.