Let's first start with how the two generations of operating systems are similar:

Note that while some of the basic features listed above are indeed similar, in general QNX Neutrino has extended the support. For example, QNX Neutrino has more POSIX support than QNX 4, simply because a large number of the POSIX specifications were still in draft status when QNX 4 was released. While less of them are in draft status as of QNX Neutrino's release, there are still more new drafts being released as this book is written. It's a never-ending game of catch-up.