Maybe the Dev team nor anyone else is aware that 4.0 is just a beta right now and can be changed very easily to break a jailbreak without having to issue a update.
also the jailbreak uses an exploit tha has been used by the dev team since 2.XX firmware onwards and apple havent fixed it also states above that apple is not aware of the exploit to even fix it, so whether 4.0 is beta or not is irelevant