A level 4 Monk would cast Wholeness of Body at spell level 2, and only regain 8 hit points.
A level 5 Monk would cast Wholeness of Body at spell level 3, and regain 16 hit points. So would a level 6 Monk.
This is because Focus Spells are automatically heightened to half your level, rounded up:
Focus spells are automatically heightened to half your level rounded up, just like cantrips are.
It's important to note that the "level" of Wholeness of Body means the spell level, not the level of your character.
Wholeness of Body gains 8 hit points of healing per spell level, as indicated by:
Heightened (+1) If you choose to regain Hit Points, the Hit Points regained increase by 8.
Per the Heightened Spells section:
Other heightened entries give a number after a plus sign, indicating that heightening grants extra advantages over multiple levels. The listed effect applies for every increment of levels by which the spell is heightened above its lowest spell level, and the benefit is cumulative. For example, fireball says “Heightened (+1) The damage increases by 2d6.” Because fireball deals 6d6 fire damage at 3rd level, a 4th-level fireball would deal 8d6 fire damage, a 5th-level spell would deal 10d6 fire damage, and so on.