You can, but the process will require alot of sacrifice eggs, as the chance of leveling your pet from 3-4 without that casher coin thingy is pretty slim.
you don't really need that cash item.
My first Lv4 pet i fused Lv3 + Lv3 + cash item but my second Lv4 pet i just fused Lv3 + Lv3 and it worked without the cash item.
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