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.

Steven387: inb4MS

Mike128: You know Itachi has MS, so Jiraiya doesn't stand a chance.