VDO.Ninja is capable of doing higher resolutions and custom resolutions however; you just need to manually specify the resolution you want. When you manually specify a resolution, if it doesn’t work, an error is thrown.
While I could make a selectable option for 4K in the user interface, another problem with 4K is that it requires a LOT of CPU power to encode. Most users will always select the highest resolution allowed, not understanding that it might actually be a bad idea. Maxing out your CPU can actually result in worse quality with lower frame rates than selecting a lower, safer, resolution.