It would still be a grimoire of 4 spells, but its customized by the player, and you only get one. it can't be traded, nor destroyed, and its one of a kind. its unique in the since that its written through magic, so the spells are imbued by the will of the owner. Of course this also means it can only be spells for the wizard spell list.
Its something that could be introduced as part of a quest, or can be a empty grimoire found in a old library, in the first region, as part of a main dlc.
(obsidian please steal this idea from me)
