Criticals and Piercer Feat

Piercer feat from Tasha’s Cauldron of Everything states that on critical attack roll you add an additional weapon damage die to the damage roll. Does this extra dice also double on a critical hit?

Ex: Critical hit with dagger with Piercer feat would be 2d4 + 2x1d4 (piercer feat) = 4d4

or is it just 2d4 + 1d4 (piercer feat) = 3d4