You normally have to wait until after you've beaten the game before you can transfer Pokemon from your 4th Generation game to Black and White; however, there are several recent event Pokemon that can be transferred as soon as you reach Castelia City, but you have to utilize a special method in order to unlock the ability to transfer them.
Getting the Pokemon Relocator
In the 11th Floor of the building in Northern Castelia City, the one on the right with the Name Rater in it, there's a scientist person up there that has developed a special program to allow the transfer of rare Pokemon. He'll only give it to you if you know the password, though!
He'll ask you for two words to start with. These are words selected from the Pokemon word library thing, initially grouped by their meaning or purpose, but you can tap the "Select" button in the bottom left corner to change it to an alphabetical search, making it much easier to find the words you'll need.
The first two words you need to tell him are:
Entering these words will cause him to ask for the second password, because all top-secret projects have to have a second password, right? Same process, but new words.
The second set of words you need to tell him are:
Voila! He'll be impressed and will then unlock the Pokemon Relocator for you! It will appear on the bottom of your game's main menu screen (Continue, New Game, Mystery Gift, etc.) as "Relocator."
Save your game in order for the option to appear on your main menu! After you've saved, go ahead and reset to get back to the menu screen.
How to Use the Pokemon Relocator
Using the Relocator can be a little tricky, mostly because you need two Nintendo DS systems at your disposal. Hopefully you have a friend's that you can borrow; otherwise, you're out of luck.
On your Pokemon Black or White version, from the main menu, scroll down to the bottom until you see the "Relocator" option. Select that and it will show a screen telling you some stuff about your other version and DS.
Now, using the second DS with Diamond, Pearl, Platinum, HeartGold, or SoulSilver inserted, turn that DS on but do not start up your game! Instead, select "DS Download Play" from your DS's menu screen and then choose "Relocator ID: XXXXX" (where XXXXX is your Trainer ID). Download the software and wait a few seconds for it to connect and download.
The Relocator will then be active and will search the 4th Generation game's BOXES ONLY for any of the following Pokemon:
•The event Shiny Raikou
•The event Shiny Entei
•The event Shiny Suicune
•The event Celebi
•The currently unreleased Lock Capsule item in HeartGold or SoulSilver
It will not let you transfer any other Pokemon or items, and those Pokemon must not be in your party (unless you don't want to transfer them).
On the DS with Black or White in it, it will show you how many distribution Pokemon you have and are going to have sent over. Verify that you want to send them ALL over (you must place them in your party in order to avoid sending them over; all held items will be placed in the original version's Bag) and then, just like that, they'll be sent to your Black or White version's PC.
These transfers unlock special events inside of Pokemon Black and White. What do they unlock, you ask?
•Transferring any of the event Shiny Crown Legendaries (Raikou, Entei, or Suicune) will unlock a special event in the Lostlorn Forest (east of Nimbasa City, on Route 16; requires Strength to get to) that lets you fight against the special Pokemon Zoroark L25. This one is always female.
•Transferring the event Celebi triggers an event when you take that Celebi to the Game Freak building inside of Castelia City (Castelia Street) and talk to the girl and boy on the first floor. As long as you have space in your party, a spare Poke Ball (any kind), and the Celebi you transferred, you'll find yourself the proud owner of a new Zorua L10. This one is always male.
Keep in mind that these are one-way transfers, so make sure you won't be missing any of those Pokemon on your 4th Generation game, as there's no way to send them back!