Can I have a code plugin on the marketplace with C++ code that is also under Creative Commons — Attribution 4.0 International — CC BY 4.0 but is consistent with Epic’s EULA?
Can you please clarify what you mean by “have”?
On the marketplace.
Items that you list as a seller on the Marketplace will be licensed to end users under the Unreal Engine EULA. If you are using code licensed under an open source software license in your plugin, you will need to submit a Third Party Software Declaration Form (See link Section 2.2e: Marketplace Guidelines - Unreal Engine).