Xbox 360 hardrive,,,,,,,,,,,,,,,?

i have a ps3 and am looking into buying an xbox 360 slime 4gb. i want the 4gb as i dont want to wait a month longer to get the 250gb one. i am only going to play maybe 3 games within the first 1-2 months as i have all the multiplayer games i want on my ps3. i was wondering if i buy the official xbox slim 250gb hard drive seperate, will i be able to fit it into the 4gb xbox? or is it not compatible because i will be buying it a month later if it does