On iOS, a mash's export quality options, including 4K, come from the snippets inside it, not from a setting.
Is the quality picker missing entirely?
If tapping export starts straight away with no quality question, every snippet in that mash is stored at 720p or smaller, so Standard is the only option, with no HD or 4K. The usual causes are Storage Optimization, which shrinks every snippet to 1280px, or snippets added long ago at a lower quality. One new snippet stored at a higher quality brings the picker back.
Is 4K missing but HD is there?
Then the largest snippet is 1080p. 4K only appears when at least one snippet is stored at 4K, and the Video Recording Quality setting affects snippets you record or add from now on—it never upgrades the ones you already have. See How to Export your Mash in 4K.
On Android, mashes export in HD (Pro) or SD; there's no 4K export.
Still missing the option?
Open the mash, tap the yellow export button, cancel, then send us your logs. They record every snippet's size, so we can tell you which one is holding 4K back.