If you like, you can return to Castelia’s sewer and check out the area up the stairs in there, since the man who lost something has moved.  You’ll find a Pokeball-shaped patch of grass of both kinds, centered around a pine tree. There’s also a Miracle Seed to be picked up here, and a man in the top left who explains that this is apparently the spot where Castelia City began.  The grass here includes several useful or interesting pokemon as well- Eevee, Audino, Pidove, Rattata, Cottonee, Buneary (Black 2), and Skitty (White 2) at levels 14-17.  Eevee is probably the most noteworthy of these, as it is rarely catchable in a game- usually you have to find it in an Item Ball lying somewhere out of the way, and you only get the one.  Having Eevee is a huge advantage, since you can leave it as an Eevee until your team is in its final setup and you know what type of pokemon you need to cover any still-open holes in your defenses or offenses.  Cottonee is a useful status-inflicting pokemon, which has abilities to support that but basically nothing else going for it- if you’re disinclined to use it, you may still want to snag one so you can trade it for a Petlil later on.  Buneary and Skitty, while both being normal-types with a high cute factor, actually fill different roles on a team.  Buneary evolves into Lopunny, a fairly quick physical attacker that doesn’t stand out much but has access to some useful capabilities- being a normal-type with powerful fighting-type attacks, for instance.  Skitty, on the other hand, evolves through a Moon Stone rather than friendship, and becomes Delcatty- a normal-type with fairly broad access to special attacks through TMs and the necessary Special stats to use them fairly well alongside a decent speed.  Both are a bit frail, but are excellent tools for attack type coverage and can be very helpful.

As you leave Castelia for Route 4, Bianca pops up in the city gate.  Rushing to catch up to you, she hands you a Dowsing Machine, which can be used to help locate hidden items on the ground.  She wishes you good luck and turns to leave, then pauses to remind you about the ability to register an item to the Y button so you can use it quickly at any time.

Route 4 is very different now from how it was in the first Black and White games, surrounded with finished buildings rather than construction sites.  For Black 2, these buildings are the ones that were being made during the first part of the ‘generation’ of games, but for White 2 they are instead the ruins that were discovered and unearthed, forcing a halt to construction.  The first things you’ll encounter here are a hiker and a Pokemon Breeder.  The hiker will point out for you that pokemon are hiding in the darker colored sand to jump out at you, while the breeder is looking for a battle so she can train the pokemon she’s raising.  Curiously, Irene will challenge you every single time you enter the route, even if you’re just returning from inside a building on it, so if you feel the need you can use her to level up some pokemon that are behind a bit, should you not like using Audino for this.

Battle: Pokemon Breeder Irene (512 Poke)
Opponent: Drilbur Lv 16, Marill Lv 16

Once you’re past the Breeder, you can see Colress to the North, in front of a series of what look to be slabs of rock blocking the road.  Take a moment first to divert to the left and head into the sandy area.