We have a warlock in our party that has a Pearl of Power. Since a Pearl of Power restores one spell slot of level 1 to level 3 only, it would seem to become useless to a pure Warlock caster that exceeds level 6, in that they can only have level 4 or higher spell slots depending on their level. Unless I am missing something.
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.
It seems a reasonable compromise to regain lower level slots before your next short rest.
As of the first set of errata to the DMG, this is now the official answer as well.
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.”
You are absolutely right.
Warlocks are unique with their spell slots. Where other classes get a myriad of slots at different levels, warlocks slots are all the highest level they can be (until they cap out as 5th level slots, after which you get Arcanums). If your slots are level 4, you wont be able to use the pearl of power to regain a spell slot.
However, your DM can certainly intervene and cause it to regain 1 warlock spell slot, which would do the trick. That's the magic gift my DM granted me and it certainly has it's uses if your party isn't willing to rest for an hour.
Still, a Pearl of Power is worth a pretty penny, so it is not entirely useless to you, and your DM may allow you to use it with the spell Identify