I think Rule 4 is valid because it is intended to catch both situations you describe: oscillations at approximately the Nyquist frequency and also much higher frequency behavior (Rule 8 is intended to catch oscillations below the Nyquist frequency).
In any of these cases, the Nelson Rules state that the signal is unlikely to be a stochastic random variable around the given mean. It is instead likely to have some underlying shape that is not being described.
The thing is, something going on above the Nyquist frequency could create ANY of the Nelson rule patterns - or none of them. Expecting it to create a nice alternating up/down pattern is wishful thinking.
In any of these cases, the Nelson Rules state that the signal is unlikely to be a stochastic random variable around the given mean. It is instead likely to have some underlying shape that is not being described.