> Proc-3 Observes: W(x),W(z),W(y), R() => y

> Proc-4 Observes: W(y),W(x),W(z), R() => z

Sequential consistency implies a total order on writes, not just that processes see the writes in an order consistent with the order each individual writer used. This history is not valid according to sequential consistency. It _IS_ valid for the two processors to see different values, if one read is stale, but it wouldn't see a different order of writes.