in Celadon city, you need the HM "cut" to access it.
Go defeat the 4th? :)
It's Erika of Celadon City.
You have to defeat Erika.
celadon city grass Pokemon
The 4th gym is in celadon city it's located at the bottom left corner of the city.
Kill yourself in the most violent way possible.
In his gym
the 5th gym is saffron gym with sabrina!
Misty
wermilion city gym badge (electric type gym)
Sabrina's gym in saffron city
Celadon City, but you must go via Cerulean -> Rock Tunnel -> Lavender -> Lavender to Celadon by pass.