I'd allow a pearl of power to take an expended spell slot of 4th level or higher and turn it into a 3rd-level slot.
Det verkar som en rimlig kompromiss att återfå lägre nivåer före din nästa korta vila.
När det gäller den första uppsättningen errata till DMG är det nu det officiella svaret också.
Pearl of Power (p. 184). The first sentence is now two sentences: “While this pearl is on your person, you can use an action to speak its command word and regain one expended spell slot. If the expended slot was of 4th level or higher, the new slot is 3rd level.”