It's not advertised but certain 4k panels can be set to 75 Hz at lower resolutions. Of course, you could always set a manual profile with a custom resolution if you wanted to.
What I will say is that no 4k panel supports using higher refresh rates at lower resolutions. The problem you're going to have with running any monitor under it's recommended resolution is that pixel scaling isn't going to be perfect and at times can even look atrocious. In the end you'll end up with a picture worse than 1080p and throwing higher refresh rates can only complicate the issue.