[Brian Bauer]

I think the decision to add Z4 or not depends on how you are doing with Z3 intervals.  Getting Z3 sessions dialed in across terrain and varying altitudes can be super tricky .  I would not worry about Z4 until you have demonstrated over time that you have mastered Z3…and something to consider is if your primary focus is ultra-distance races, what do you hope to get form Z4 workouts which will either be run extremely fast, or on extremely hard uphills.  I work my sessions at the top end of Z3 only( not Z4), and my races like skimo, nordic and trail running( under 30k) definitely include red-line hammer segments…and knowing what they feel like and what recovery between efforts requires is really important.