Walkthrough
After defeating the Pewter City Gym, you will earn access to Route 3 just east of the town. There are loads of Wild Pokemon to be found here, as well as a number of Pokemon Trainers looking to battle. Route 3 will take you directly to Route 4.
Wild Pokemon
Grasses:
Spearow (35%): Levels 6-8
Pidgey (30%): Levels 6-7
Nidoran Male(FireRed)/Nidoran Female(LeafGreen) (14%): Levels 6-7
Jigglypuff (10%): Levels 3-7
Mankey (10%): Level 7
Nidoran Female(FireRed)/Nidoran Male(LeafGreen) (1%): Level 6
Pokemon Trainers
Lass Janice: Level 9 Pidgey, Level 9 Pidgey
Bug Catcher Colton: Level 10 Caterpie, Level 10 Weedle, Level 10 Caterpie
Yougster Ben: Level 11 Rattata, Level 11 Ekans
Bug Catcher: Level 9 Weedle, Level 9 Kakuna, Level 9 Caterpie, Level 9 Metapod
Lass Sally: Level 10 Rattata, Level 10 Nidoran Female
Bug Catcher James: Level 11 Caterpie, Level 11 Metapod
Youngster Calvin: Level 14 Spearow
Lass Robin: Level 14 Jigglypuff
Pokemon Trainers - VS Seeker Rematch
Lass Janice: Level 20 Pidgeotto, Level 20 Pidgeotto
Lass Janice: Level 28 Pidgeotto, Level 28 Pidgeotto
Bug Catcher Colton: Level 19 Metapod, Level 19 Weedle, Level 19 Metapod
Bug Catcher Colton: Level 27 Butterfree, Level 27 Kakuna, Level 27 Butterfree
Bug Catcher Colton: Level 51 Butterfree, Level 51 Beedrill, Level 51 Butterfree
Yougster Ben: Level 17 Rattata, Level 17 Ekans
Yougster Ben: Level 28 Raticate, Level 28 Ekans
Yougster Ben: Level 48 Raticate, Level 48 Arbok
Items
Oran Berry: ItemFinder