Not sure it would be easy to restore 1/4 of 1, 2 or 3 spellslot... That's why I finally only suggest full/half but anyway.
Good point ...
Maybe i should also add minimum 1, after all those words are used quite often in PHB.

Also, i believe you get exactly same problem with 1 or 3 spellslots no matter if you divide them by 2 or 4.
