Yes, it would reduce the search space three-fold (the 4th mechanism uses the same stored hash as the 1st), and your suggestion of different salts would prevent

Yes, it would reduce the search space three-fold (the 4th mechanism uses the same stored hash as the 1st), and your suggestion of different salts would prevent that since each password variant would need a separate search. Still, the difference between looking for 3 things out of ~10^20 options vs. just 1 thing is not that big.