I think you have to go back to Hearthome city. There you can enter the gym and fight Fantina.
pastoria city
The forth gym leader is in Veilstone City
The first gym in Pokemon diamond is in Oreburgh City
The Pokémon the second Gym Leader has in "Diamond" are Cherubi, Turtwig and Roserade.
The 6th gym leader is in Canalave City.
you beat the gym leader
He is in the lighthouse
It is Candace, The Ice-Type Gym Leader.
Go to the lighthouse and the Gym leader should be there.
by winning a contest and by defeating the pastoria gym leader
just beat the fourth gym leader
a gym leader can't teach a Pokemon that move but you have to defeat the 6 gym leader in order to teach that move