it means: I'm
This seemingly cryptic looking term is actually the word "I'm" (the contraction of "I am"). When you see "IB4m" online, it's most likely because the statement "I am" has been distorted due to different computer programs rendering the apostrophe differently. In other words, the reason this term appears as IB4m instead of I'm is because of gremlins. Gremlins are mysterious characters that sometimes appear on your computer screen, in text documents, or in e-mail messages. For example, if you type a word with an apostrophe using Microsoft Word and then copy it over into HTML, the apostrophe may appear as a small box instead.
The same thing happened years ago when this code " & n b s p ; " suddenly appeared all over the Web (which is actually the HTML code for the "&" sign). Some browsers didn't interpret the HTML code for the "&" sign correctly so they displayed the actual cryptic looking code.
FYI: I've seen several forums where people from different countries post questions and it appears that the computer software running the forum is interpreting "I'm" as "IB4m" I also saw "I'd" rendered as "IB4d" (example link below).
NetLingo Classification: Online Jargon