Released versions of QNX 4 are strictly single-processor, whereas QNX Neutrino has support for SMP.
SMP is a great feature, especially in an operating system that supports threads, but it's also a bigger gun that you can shoot yourself in the foot with. For example, on a single-processor box, an ISR will preempt a thread, but never the other way around. On a single-processor box, it's a worthwhile abstraction to “pretend” that threads run simultaneously, when they don't really.
On an SMP box, a thread and ISR can be running simultaneously, and multiple threads can also be running simultaneously. Not only is an SMP system a great workstation, it's also an excellent SQA (Software Quality Assurance) testing tool—if you've made any “bad” assumptions about protection in a multithreaded environment, an SMP system will find them eventually.