In the theme park in Nimbasa City.
the fourth gym is in celadon city at the bottom left of the city. the gym leader is erika. the badge is called the rainbow badge
Beat the Gym Leader
Go to the forth gym!
The 4th pokemon badge is given by Erika the Celadon City Gym Leader.
misty
by beating the fourth gym leader
Flannery's gym in Lavaridge Town.
in the museum
The fifth gym leader in Pokemon platinum is crasher wake but you get the fourth badge.
Flannery, a fire type gym leader.
Directly to the right of the city with the fourth badge.
Defeat the celadon city gym leader.