In the Lavaridge Town gym.
The 4th gym is found on Lavaridge town and the gym leader is Flannery.
The leader is Falnery, she uses fire type pokemon.
dark type
In sapphire, the 5th gym leader is your dad, in that city after Oakdale.
The 6th gym leader in Pokémon Sapphire is found at the Fortree Gym. The name of the gym leader here is Winona.
You have to Dive and then you can battle the Gym Leader.
The 8th Gym Leader in Pokémon Sapphire can be found in Sootopolis City.
The third gym leader in Pokemon Sapphire is Wattson and he is located in Mauville City.
I believe that is after you've beaten the 4th Gym in Volcanoridge Town.
In Lavaridge gym.
There in the gym in mossdeep city by the way the gym leader are actually gym leaders.
After defeating Flanery in the 4th gym, go to Petalburg City (the 3rd city) to fight your dad in the 5th gym.