In Veilstone City.
The forth gym leader is in Veilstone City
it is in pastoria city
The fourth gym is in Ecruteak City. It is a ghost type gym and the leader is named Morty.
pastoria city
It is Crasher Wake, in Patoria City.
Crasher Wake is in Pastoria City.
Simple use a water, ground or rock type Pokemon at the fourth gym
I think you have to go back to Hearthome city. There you can enter the gym and fight Fantina.
If its the game im thinking of then Lieutenant Surge
Ghost, but they're poison too
beat the forth gym leader
Galactic Grunts are everywhere! They play a major part in the game.