I always laugh when I see a z4 workout prescribed that is maybe 4x8 or something. That's barely more than Vo2 and for pros, they probably do do 32 minutes or more of Vo2.
I agree with your point on overtraining. Yes z4 is technically defined by 1 hour max, but actually doing that is a lot of stress so I wouldn't want to do 2x40 too often, or at all. I started my z4 block earlier in the year with 2x15 then 2x20, 2x25, to 2x30 which I did a few times.