Western segment
Route 4 is a uniquely designed route that contains two sections split up by Mt. Moon. On the western portion of the route, there is the western entrance to Mt. Moon, as well as a Pokémon Center. Inside this Pokémon Center, there is a shady man who sells the player a Magikarp for $500. He is only present in Generations I and III.
The western portion of Route 4 begins by traveling north from the previous route; Route 3 towards the mountain. There are several small ledges spanning from both sides of the route, however, there is enough clearing in the center to pass through. There are also several rocks along the sides that apparently can be mistaken for Geodude as a little girl explains as she trips on one. By traveling straight on, ahead will be a small Pokémon Center, one of two Pokémon Center's that can be found on a route, the other being on Route 10. The western terminus to Mt. Moon can also be found by traveling west of the Pokémon Center, which has been indented into the mountain face.
In the Pokémon Center, Trainers can heal their Pokémon after a hard journey through Route 3, however, a number of other things can be done in the Pokémon Center. In Generation I and III, a shady man, which happens to be the Magikarp salesman can be found in the corner of the Center selling Trainers a Magikarp for $500. He does not offer a refund. While this may seem something of a ripoff, some players consider it very worthwhile to raise a Magikarp from such an early point in the game, allowing the player to train and evolve it into a Gyarados much earlier than if they had waited until they are given an Old Rod. There is also a man reading a newspaper, reading that Team Rocket are attacking Cerulean citizens.
Eastern segment
From Mt. Moon, Route 4 travels to the east, swerving to the north and the south to avoid ledges along the route. The path turns north, then east near a large clump of trees surrounding a grassy area. The route is a very strange route as the ledges form a contour down the mountainside, leading Trainers due east and then south to the end of the path and a ledge that will force Trainers to go down it into a large clump of trees surrounding a grassy area. After going down this ledge, there is no return and Trainers must go east to reach Cerulean City.
Route 4 can, in fact, be reached from Cerulean City, by using Surf, Trainers can Surf along to western part of Route 24, south to Cerulean Cave. Trainers can then travel west, past a Lass on the highest hill, and then jump down the remaining ledges to access this section of the route.
Due to a ledge that separates Mt. Moon from Cerulean City, Trainers cannot return to the previous towns until they travel through Diglett's Cave and use Cut on Route 2, use Fly to return to the cities west of Mt. Moon, or use Surf down from Route 24. The majority of the route is fairly straight-forward as there is only one true way to proceed on the path and there are no Trainers to battle (with the exception of the Lass on the highest hill on the route; however, she is reached only by way of the waterway that begins on Route 24).
Depending on the Generation, there are items, Move Tutors, or both along this path. In Generation III, two Move Tutors were added to the route. Located near the Mt. Moon entrance, these tutors will teach a Pokémon the moves Mega Punch and Mega Kick. Both tutors are jealous of the other's skills and will try to get Trainers to come to them instead of the other.
Generation I and III
Route 4 endured minor cosmetic changes but was not unchanged. The "gates" or the gaps in the ledges used for passed up them on the eastern segment were widened significantly, while the patch of tall grass near the Cerulean City entrance has grown in size covering the entire area. In the western segment, rocks and boulders had been included in Generation III for visual effect. Apart from this, most aesthetic features have been enhanced. Trees have become bigger, flowers are more visible, and most other sprites have been upgraded to Generation III standards.
Aside from cosmetic changes, two Move Tutors were added to the east portion of the route. Located near the Mt. Moon entrance, these tutors will teach a Pokémon the moves Mega Punch and Mega Kick. Both tutors are jealous of the other's skills and will try to get Trainers to come to them instead of the other. Other than this, only item positions and types were changed. TM04 (Whirlwind), located on the Second-highest hill was changed to TM05 (Roar). The Great Ball hidden in the west segment was replaced by two Berries; a Razz and a Persim Berry in similar locations.
Generation II
In Generation II, the western portion of Route 4 was cut off and changed to an extension of Route 3. Due to this, Route 4 western entry starts at the eastern entrance of Mt. Moon, although most of the eastern portion remains the same. Along with this, the route was shortened significantly as the length between Mt. Moon and Cerulean City was slashed in half, as was the grassy area, however the layout and positioning of ledges and signposts remains mostly the same. There is no entrance to the Cerulean Cave from this route and the road is blocked off by barriers. The TM was replaced with an HP Up and three Trainers were added to the route.
From Mt. Moon, Route 4 travels to the east, swerving to the north and the south to avoid ledges along the route. The path turns north, then east near a large clump of trees surrounding a grassy area. The route is a very strange route as the ledges form a contour down the mountainside, leading Trainers due east and then south to the end of the path and a ledge that will force Trainers to go down it into a large clump of trees surrounding a grassy area. After going down this ledge, there is no return and Trainers must go east to reach Cerulean City.
Generation IV
Other than graphical updates, the basic layout remains the same as previous generations, but the route is no longer flat, with hills leading down from Mt. Moon to add more depth to the route. Like Generation II, the western portion of Route 4 was cut off and changed to an extension of Route 3. Due to this, Route 4's western entry starts at the eastern entrance of Mt. Moon. Items and Trainers remain the same as in Generation II. Unlike in Generation II, Cerulean Cave remains in Cerulean City, however, unlike Generations I and III, there is no return to this route from the cave, likely to prevent the player from traveling to west Kanto without restoring the Power Plant.
