Generally used on internet forums, inb4 refers to a user posting a reply to a message/topic "before" another user posts an obvious response. When used appropriately, inb4 is followed by a word or short phrase that the user knows will eventually appear in the topic conversation.
Jonny113: Jiraiya would beat the crap out of Itachi.
Mike128: You know Itachi has MS, so Jiraiya doesn't stand a chance.
An abbreviation used on messageboards like /b/
in anticipation of a certain image, event, or meme occuring.
"In b4 Togepi" = I am sure that any second now, a picture of Togepi will be posted to this thread.
"In b4 b&" = There is no doubt that someone is about to get the banhammer
because of their churlish behavior.
"In b4 v&" = I am quite certain that the FBI will soon be showing up to take us all for a ride in the party van
, and if such a thing happens, you can't say I didn't see it coming.