I still have to criticize Nvidia. I don't know if this has been done before on previous cards or not (which doesn't make it ok), but when you advertise a certain amount of memory with no indication that part of that memory performs differently/worse, then that's just wrong.
Would it be okay if 0.5GB of the 4GB was the pool that actually ran at full speed? I mean technically you still get 4GB... no sane person would say yes. It's not much different than what is happening with the GTX 970 and I think Nvidia should totally be accountable for it.