The user of a wand is the caster of the spell and the wands creator makes all the decisions except targeting for the spell contained within. A scroll, on the other hand, is cast in its entirety, on the spot but still treats the reader of the scroll as the caster. This leads to a couple of conclusions.

For wands;

The activator of the wand can only choose the target for imbue with spell ability. As such, the spells to be imbued must have been determined when the wand was created and must, therefore, also be contained within the wand. It strikes me as unreasonable for those not to come with a commensurate cost based on their level(s) but that's not RAW.

Alternately, the wand produces the effect of imbue with spell like ability and the takes the imbued spells from the user's spell allotment as was decided when the wand was created, if he has them prepared or knows them and has the appropriate level slots. If he does not have them prepared or does not know them, the spell is still cast but has no meaningful effect.

For scrolls;

Since all decisions about the spell are made as the scroll is being used and that must include the spells to be imbued, such a scroll is useless to someone who does not have any spells to imbue.


In either case, a GM could rule that since the item bypasses the normal ongoing cost of consuming one of your 4th level slots, that the spell cannot be made into such an item. Alternately that using the scroll does -not- bypass that requirement and that any use of the item consumes both the item or its charge -and- one of your fourth level slots, sacrificing one of the spells you have prepared in such a slot, if any.