i'm not sure how likely it is for someone to find this and suggest a 4tb external as it's kind of uncommon i would say. they look pretty expensive (on newegg, at least) in terms of backup, but I guess that's what you pay if you need 4tb of storage. is it really necessary, though? that seems like an awful lot. i also doubt that you will find something slim, they are all looking like bricks to me.
i'm sorry i can't offer any real help - just offering my 2 cents and asking whether or not it is really necessary for that much storage? (esp considering this being posted in the mbp forum)