It add four attack EVs.
>Promotes Attack gain on level up, but halves Speed stat in battle. Adds 4 Attack EVs whenever experience is gained in battle, regardless of the Pokémon fought. Its effect is doubled by Pokérus. -Bulbepedia
So you would receive a total of 5 attack EVs, if you were fighting a Pokemon that yeilds 1 attack EV. Otherwise, it would be 1 EV and 4 attack EVs.