Incidentally, Bell Labs patented using a NOR gate to reject dis-allowed states in ring counters in 1987:
Given that most D flops have not-Q that's all the patent is, a NOR gate connected from two outputs to clear. for 20 years until 2005 you'd need to ask Bell Lab's permission if you wanna do that! Heh!
Anyway the point is when using discrete flops to build one, perhaps in different IC packages even, I think actively rejecting dis-allowed walks is important thing.
for a four-flop counter I think a quad-input XOR can generate a reset for the dis-allowed states just by "inspection." If your outputs are ever 1010 or 0101 it's gone sideways.