Hey there @d_maciel2 , thanks for getting in touch about this. I understand you've also faced this issue of black bars when playing at a 4K resolution, which is most unusual. I'd really like to help you solve this.
As far as I can see, we've had no other reports of this issue. I'd imagine our teams routinely test our games on 4K displays, to check for issues. For this reason, we've asked for a screenshot so that we can pass it on internally - this will help our teams to replicate the issue on our end.
Before we do that, though, can you make sure to run through our troubleshooting guide? The steps we've listed have been shown to fix the vast majority of graphical issues, so they are well worth performing. In particular from the guide, please make sure you are running the game with administrator rights, you've verified the game files, you've got your graphics drivers and Windows install fully updated, and you've (at least temporarily, for testing) deactivated all background programs.
If the issue persists after that, can you try again to send that screenshot across to us? Rather than uploading it directly to the forum, it'd be great to have it uploaded to any other image- or file-sharing service of your choice, and then send us a link here in the thread. It'd also be really useful to have some screenshots showing the graphics settings you've chosen in-game, particularly the resolution.
For refunds, please see our Ubisoft Store refund policy. The linked article contains links in turn to Steam's and Epic's refund policies, too.
Please let me know if I can clarify these steps further. Thanks again for your report!