* It's not clear whether you are talking about your CLI-221-Black dye cartridge or PGI-220 pigment black cart. Also, is the MediaType set to "Plain"?
* Theoretically, a printer could allow b/w text to print when a dye cartridge is empty, but "it is what it is". Also, there are so many things that could do wrong with "corner cases", that I find myself in agreement with Canon's restrictions.
* They are doing us a favor to reduce "shooting ourselves in the foot". We can agree to disagree on that, and YMMV.
* The iP4700 is a pretty decent printer with separate CMYK dye carts, and separate pigment cart. IMO, it would seem to be a shame to potentially or actually damage it by doing what the other replies describe (and I realize that neither are advocating what they describe, and providing accurate answers to your question).
* If you really just want to do b/w text printing, a monochrome laser could be a much better option than pretty much any inkjet.
* To me, disabling ink level monitoring as described in the second reply is a really last resort, to be avoided more than 99.44% of the time. I'm not a mind-reader, but I don't think drdoug is advocating that you do what he describes. Hope not, anyway.
* Your warranty is probably already expired, but disabling ink level monitoring will void the warranty, which is completely reasonable on Canon's part, imo.