After landing on Akala Island and exploring Heahea City, you continue on to Route 4.
Go through the grass and you will battle Bellhop Jody. He has a level 14 Fomantis. You get 560 Pokédollars for winning.
If you take the lower path through the grass, you find a Great Ball.
Go north, then go east in a narrow path and check on the rock at the end to get a Tiny Mushroom.
Then go to the left and go up the ramp. Go west through the grass to battle Collector Bryan. He has a level 14 Munchlax and a level 14 Bagon. You get 448 Pokédollars for winning.
Afterward, check behind Bryan to get a Revive.
Go back through the grass, then go upward up the ramp. You can battle Sightseer Scotty up above. He has a level 14 Rattata. His Rattata is not the usual Alola form, so this form will be added to your Pokédex with this battle. You get 840 Pokédollars for winning.
Near Sightseer Scotty, you can find the entrance to Pikachu Valley to the north.
When you enter Pikachu Valley, you will get a Pikachu frame to use on photos you have taken at the Alola Photo Club.
Check behind the RV to find a Magnet. If you try to go into the RV, a Pikachu will come out. You can keep doing this, and the guy next to the RV will tell you how many he has seen come out.
Talk to the girl in the upper-right corner to take a Pikachu quiz. If you answer all five questions correctly, you get a Pikachu Shirt and Pikachu Cap that you can change into from a changing room in an apparel shop.
Along the river here, there is a Poké Finder spot where you can take photos.
Route 4 Continued
After you leave Pikachu Valley, go to the right and go down the one-way ledge to find an Adrenaline Orb.
Then go north through the grass, and go west if you want to battle Sightseer Perdita. She has a level 14 Meowth. It is not the usual Alolan type, so this form will be added to your Pokédex. You get 840 Pokédollars for winning.
Go east down the ramp. When you find some grass, go east through it. There is another patch of grass after that. Go south through that grass to get an Energy Root.
Northeast from there, you can battle Cook Ernie. He has a level 14 Cutiefly. You get 560 Pokédollars for winning.
Go north from there to reach Paniola Town.