waste pipe 3/4" below concrete

[rancouw]

I'm remodeling a bathroom and after taking out the toilet, old tile and toilet flange, I found that the 4" pvc toilet waste pipe sits about 3/4" below the surface of the concrete. The concrete is chipped out around where the old flange was sitting but the cut out is shallow enough to still accept 2 1/2" tap cons. Do I need to extend the waste pipe or can I just extend the concrete up by putting a temporary 4" collar as a space holder then, after the new concrete cures, use a toilet flange with a longer pipe to glue into the existing waste pipe?