Also, on your way out of town, make sure you pick up the Eviolite by talking to the Scientist in the second building to the right. You need to have seen 40 Pokemon in order to receive it, so you weren't likely to before heading into the sewers, but now you should have seen enough to receive it. Equip that on any of your Pokemon that are still capable of evolving and they'll get a +50% bonus to both their Defense and Special Defense, making it an awesome item for whichever of your unevolved Pokemon wears it.Make sure you heal up your Pokemon if they're weak, because you've got another fight coming up fairly soon. Head over to the Central Plaza on the northern end of town and you'll run across that fellow you ran into earlier in the sewers. He'll approach you and ask if he can see your Pokemon. Say Yes and he'll examine your lead Pokemon, telling you that it overflows with confidence. He'll introduce himself as Colress and tell you that his theme is "What do Pokemon draw their power from?" Lastly, he'll ask if he can check in a battle, so say Yes; however, he'll say he wants to do so on Route 4, so go ahead and follow him over there.
Now, head onto Route 4 and get ready to face Colress!
The entire layout of Route 4 changes drastically depending on which version you're playing! In Pokemon Black 2, there's a lot more urban sprawl with buildings and such dotting the straightforward path forward, but in Pokemon White 2, you'll have to take a left instead and venture around a new path.
To accommodate this, I've added two sections: Route 4: Black 2 and Route 4: White 2. Read the appropriate one for your game for instructions on how to get through.