I think we are totally on the same page here and it’s just one thing in your wording here - “as hard as you can hold, for the 4 mins. regardless of the next one” is having me confused but everything else you’ve written here we seem to be in agreement that all the intervals should be substantially equal in power. Which is totally what you are doing in your illustrated intervals!
And your last comments are right on as well. My Z3, Z4 and Z5 zones are 40 Watts across! You didn’t fail your workout if you didn’t keep them all at exactly the same number.
I’ve heard a few coaches talk about using the performance of the intervals as the determiner for the number to do! Just keep doing them until you have one that doesn’t go well. I’m a little too type A to be that flexible. This is also how some weight lifters do it. They pick a weight and number of sets, but the reps are done by feel - to some reps in reserve or to failure - but not to a specific number.
I’m hoping newer cyclists are finding this useful, these are great discussions! The right workout is the one that you’ll do. All these studies with specific intervals and zones and execution - these are all simply things that have been studied that need rigorous protocols and show results based on statistical significance. But if you look closely at the individual results of these studies - they are all frequently all over the place. 1 athlete sees a 40% improvement, 5 see a 10% improvement, 2 see no improvement, 1 sees a 15% decrease.
AFAIK all of this is a starting point for you to try different workouts, see what you like, see what makes you improve.