I believe the forth gym in Blue is Erika of Celadon City. All you need is HM Cut, which you get in Vermillion City. You may also have to defeat Team Rocket in the Celadon City's Silph Co., but I'm not sure.
The first gym is in pewter city.
The 5th gym is the Fuchia city gym Gym leader Koga
get the 4th gym badge and then give him a pokemon... get the 4th gym badge and then give him a pokemon...
Go in. Beat the GYM. THAT SIMPLE
in veilstone city
at the daycare at the nearly at the 4th gym
In Pokemon Blue, the fourth gym a player may battle is in Celadon City. One of the player's Pokemon must know the HM Cut in order to access the gym which is a Grass-type gym.
hgfjfjtjjkyjtt
The first gym is in pewter city.
The 5th gym is the Fuchia city gym Gym leader Koga
go to the 5 gym
canalave city...
his gym
get the 4th gym badge and then give him a pokemon... get the 4th gym badge and then give him a pokemon...
you got to beat all the Pokemon gym leaders in kanto other then blue,then talk to him
In Pokemon platnium it is wake gym leader. P.S. he has water type Pokemon.
go to the 4th gym city market at 5th floor or 4th and buy from the 2 shop keeper