Route 4

When you get to the gate, Bianca will battle you. She has:
Herdier Lv. 18
Muuna Lv. 18
She will have these two no matter which starter you picked.
Snivy picked:
Pansear Lv. 18
Dewott Lv. 20
Tepig picked:
Panpour Lv. 18
Servine Lv. 20
Oshawott picked:
Pansage Lv. 18
Pignite Lv. 20
Herdier has Intimidate, so watch out for that. Be wary, because she also has 2 Super Potions.
She should be no problem. If your Pokemon are weak, HEAL NOW! You'll have to battle Cheren next. After healing, go north and battle Cheren!
He has:
Pidove Lv. 20
Liepard Lv. 20
no matter which starter you have.
Snivy picked:
Pansage Lv. 20
Pignite Lv. 22
Tepig picked:
Pansear Lv. 20
Dewott Lv. 22
Oshawott picked:
Panpour Lv. 20
Servine Lv. 22
His starter is holding a Sitrus Berry. Then, there's the sandstorm that will damage any Pokemon not Rock-, Steel-, or Ground-type, so take that into account.
After beating him, you'll get an Xtransceiver call from the Prof., asking you to go to Nimbassa. Cheren leaves, but we don't have to go there just yet.

SAND
Sandile (40%)
Darumaka (40%)
Scraggy (20%)

There is a Hiker staring at an X Accuracy in the middle of a bunch of sand, where apparently wild Pokemon appear in! It does not matter if you go down or up. I reccomend going up, because there is less sand, and a house where you can heal up. Down the stairs are two fishermen; the one farthest down has four Basculin Lv. 16, the other has 3 Basculin Lv. 18. Head north of the stairs to find a Parasol Lady with two Tympole Lv. 18. Even farther north is a Worker with two Roggenrola Lv. 17 and a Timburr Lv. 17, then north throug hthe sand is yet another worker with a Timburr Lv. 19. Up the stairs is nothing, so head north to get TM41 Torment. Back to the main road. Oh yes, and by the Parasol Lady is a Hyper Potion.
Right after the road stops, you find a Burn Heal, and a Backpacker with a Darumaka Lv. 19. Off to the west is a Backpacker with a Cottonee Lv. 19. And up north, the Backpacker has a matching Petilil Lv. 18. Next to her is a Super Potion and a Backpacker with a Blitzle Lv. 19. Keep heading north to find the last Backpacker just past a freeway, with a Pidove Lv. 19. If you want to explore, I'll cover that now.
Go south of the freeway, then west, and you'll find a Worker with a Roggenrola and a Timburr, both Lv. 18. The Worker past him has a Timburr Lv. 18. In the house a guy will give you TM28 Dig. Further north is the Desert Resort.
That's just about it, so either go to Nimbasa or the Desert Resort (which is optional at this point).
GO TO DESERT RESORT
JUST GO TO NIMBASA CITY