It's only an issue if that last 3db of SPL is making all the difference for you. And that would be if the NAD put out twice the power at 4ohms (i.e., max. voltage stayed the same). Many manufacturers do not rate their amp at TWICE for four ohms, but some reduced rating instead.

It seems NAD is (also) saying, "No point in doubling the power at 4 ohms, it's a barely noticeable difference, and even then only when wide open. This way, the 4/8 ohm speaker decision is not a factor when using this amp."