From my understanding of how these whitelists work, Kodi will directly output the media in 4k if the 4k mode is whitelisted (but I'll check my logs to be sure). You can also do the same with lower modes and it will directly output in 720p for example. There's a bit of information about it in the PR for whitelists:
Mode whitelist by lrusak · Pull Request #13274 · xbmc/xbmc · GitHub
Edit: And if you haven't set a 720p whitelist, it kodi should upscale to whatever is set in settings (1080p)