My personal trainer/indoor sessions don't involve Zone 4. The zones I pay particular attention to in my sessions are Zones 2 & 3 (Aerobic) & Zone 5 (VO2 Max 90%-100%). There is nobody on earth who can hold a 4-5 minute stint @ 100% VO2 Max if I'm reading you right. Think of the 100m sprint.
You ain't holding that 5 minutes thats for sure.

My stints @95+%-100% VO2 Max all out effort, build up from 20 seconds, then back right off the power & back into the easy zones 2 & 3 for 3-5 minutes, then go again into zone 5, building upto 50secs-1min intervals in zone 5 during a session. As I'm building upto 1 min, the % is ever decreasing from start to finish but I'm trying to hold it within the 90% range.

Had good success with this. Easy to hard, easy to hard, nothing is constant. Want to increase your power output that is how to do it. I probably do tons of zone 4 out on the road anyway, in fact I know I do, but on the indoor trainer, different goals, alot more grimacing indoors.

That is what I do personally, thats what I believe in. It can get tough at times, but where involved in a tough sport.

As long as your cranking.