I'm assuming you mean you have a 2+ armour save
Ap modifies the dice roll you make, and yiu then compare that to the armour save you needed to make. So ap-4 has a similar effect to reducing a 2+ save to a 6+ save, but doesn't do so. It takes a d6 and reduces the value of that dice roll by 4, so only a 6 or more will, after deductions, become a 2+.
You seem to be recalling an earlier edition, where the AP value indicated which value of armour save (or worse)the weapon entirely ignore. So AP4 would ignore 4,5 and 6+ saves entirely.
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