Yes, you're asking for something out of "core" of h264 spec.
It only recently extended profiles covering 8K (so 3840 height) were added to h264 spec and Resolve exporter doesn't support them (well- not many software will most likely).
You need to export some intermediate file (ProRes, DNxHR, Cineform etc) and then use e.g. ffmpeg with h264 or h265 codec. If I'm correct ffmpeg should export h264 at this resolution. You may be actually better of with h265 with such a resolution.