You have to Dive and then you can battle the Gym Leader.
The fourth gym is in Lavaridge the 6th is in fortree.
the fourth gym its a fire gym
Your dad in Petalburg City
The 4th gym is found on Lavaridge town and the gym leader is Flannery.
in Lavaridge town, below Fallarbor town
go to the desert get the fossil and then go to petalburg city and defeat the gym there
defeat the fourth gym leader and when u come out of the gym, may will give u the go-goggles.
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.
It is Petalburg City. That is one of the first towns you go to in the game and is the gym that your dad is the leader of. He uses Normal-Type Pokemon.
After defeating Flanery in the 4th gym, go to Petalburg City (the 3rd city) to fight your dad in the 5th gym.
The 8th Gym Leader in Pokémon Sapphire can be found in Sootopolis City.