Charlaquin
Goblin Queen (She/Her/Hers)
That’s not correct. The wording of PAM is “When you take the Attack action and attack with only a glaive, halberd, quarterstaff, or spear, you can use a bonus action to make a melee attack with the opposite end of the weapon. This attack uses the same ability modifier as the primary attack. The weapon's damage die for this attack is a d4, and it deals bludgeoning damage.”Agreed on 1d4 -- the attack does not use the weapon's damage die in its calculation. As written, PAM does 1d4+STR -- even doing 1d4+WIS requires a judgement, but it is a reasonable judgment to make and (I think) within the parameters of the feat.
No mention of Strength (so as Jeremy Crawford says, you could use Wisdom for the attack and damage if you had Shillaleagh cast on the weapon), and it does indeed use the weapon’s damage die; it just changes the weapon’s damage die to a d4 first.
The reason PAM’s damage die replacement wins over Shillaleagh’s damage die replacement is that it’s more specific. It affects one specific attack with the weapon, where Shillaleagh affects the weapon in general.