Walkthrough
Route 4 is found at the end of Route 3, and it's an extremely short path that is divided into two sections because of Mt. Moon located in the center of it. The Pokemon Center found within Route 4 is located on the western side, while a few Move Tutors can be found on the other end of it. There are a few Wild Pokemon here.
Magikarp: Can Be Purchased For $500 From Man In Pokemon Center
Mega Kick & Mega Punch: Can Be Found On The Eastern Side Of Route 4
Wild Pokemon
Grasses:
Rattata (35%): Levels 8-12
Spearow (35%): Levels 8-12
Ekans(FireRed)/Sandshrew(LeafGreen) (25%): Levels 6-12
Mankey (5%): Levels 10-12
Surfing:
Tentacool (100%): Levels 5-40
Magikarp (100%): Level 5
Horsea(FireRed)/Krabby(LeafGreen) (60%): Levels 5-15
Krabby(FireRed)/Horsea(LeafGreen) (20%): Levels 5-15
Magikarp (20%): Levels 5-15
Horsea(FireRed)/Krabby(LeafGreen) (84%): Levels 15-35
Gyarados (15%): Levels 15-25
Psyduck(FireRed)/Slowpoke(LeafGreen) (1%): Levels 25-35
Pokemon Trainers
Lass Crissy: Level 31 Paras, Level 31 Paras, Level 31 Parasect
Items
TM05: On Ground
Persim Berry: ItemFinder
Great Ball: ItemFinder
Razz Berry: ItemFinder