I'm assuming what you mean by "tabs 3 & 4" are "Key Items" and "Crystals".

Key Items and Crystals are not inventory space and those slots cannot be assigned to regular inventory. The Key Items tab looks like it has a lot of empty slots but in reality that is only a style choice of the UI. The Key Items space is always exactly as large as it needs to be for the currently held key items; those empty slots do not exist. The Crystals tab doesn't store any item information; it's just the quantities of each crystal.