That would be Erika the grass type trainer.
The fourth gym leader is Flannery.
The fourth gym leader is Maylene in Veilstone City, a fighting type gym leader.
In the Lavaridge Town gym.
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go to hearthome city and fight the gym leader there
go to the 5 gym
In Pokemon platnium it is wake gym leader. P.S. he has water type Pokemon.
The fourth gym leader in Pokemon Soul Silver is in Ecruteak City.
Flannery
fatina (hearthome)
The Gym Leader that allows you to use the HM move Strength in Pokemon FireRed is Erika. Erika is the Gym Leader of Celadon City and is the 4th Gym Leader the player faces.
The 4th gym is found on Lavaridge town and the gym leader is Flannery.