Typically we introduce the Z3 work as a precursor to the Z4 intervals. This depends to a large extent on your response to this higher intensity. Read the quote from Per Aastrand on Page 170 of Training for the Uphill Athlete to see that there is no one size fits all for the prescription of the use of high intensity training. There is a great deal of info in that book on how to introduce intensity.
One word of caution I would not add ME and Z3 and Z4 workouts in the same period. Again this is explained in the book. You run a serious risk of overtraining by doing so. This is why, for runners and Skimo we periodize. We use the ME in the base period as a preparation for the high intensity training to follow. We do this even for athletes with ADS and have seen very positive results doing so.
I hope this helps.