Route 4 is a short route that connects Mt. Moon to Cerulean City. Four items can be found here. First, jump the ledge to the south of Mt. Moon to find a Paralyze Heal.
Second, up the path to the northeast are 5 Poke Balls on a small hill. Third, a Repel can be found in a corner to the southeast.
A ledge to the east will cut you off from going back to Mt. Moon and Pewter City for a short time, so make sure you’ve collected everything and captured the Pokemon you like before making the jump!
The patch of tall grass with wild Pokemon are on the other side of this ledge, which then leads to Cerulean City. If you look close enough, you can find 3 Great Balls behind some shrubs to the east of the patch of tall grass.