Neutrino totally redesigned the way that the operating system was embedded. Under QNX 4, in the original release, it was marginally embeddable. Then Neutrino came along, designed to be embeddable. As a bonus, QNX 4 underwent some changes as a result of the experience gained in Neutrino, and now QNX 4 is vastly more embeddable than it had been. In any event, embedding QNX 4 versus embedding Neutrino is almost like night and day. QNX 4 has no real support for things like:

whereas Neutrino does. The definitive book on that subject is QSS's Building Embedded Systems.