I know that I feel like I really have around 4-5 good hours in me of serious deep contemplative programming a day. After that I either check out or find myself doing menial tasks that require no brain power for the rest of the day.
If I do have a huge deadline and have to double or triple that in a day I find myself mentally wiped and kind of useless for several days there after.
Yes, from your point of view you got "4 hours" of real, valuable and productive work in you in any given day.
But those nagging administrative tasks, status reports, talking to project management professionals in pro-forma meetings. THOSE are considered work and productivity for the people that set these up. There are job-roles for which meetings themselves are work-- while that might be a vision of hell for many of us, it is reality for layers of management and their minions.
My concern with this concept of 4-day workweeks is that it will precipitate evenmore tracking of productivity and even more meetings to gauge progress at the organizations that are trying to convince themselves to switch.
There's something to be said for having the slack to figure stuff out at work without being tracked constantly. A shorter work week is likely to increase "time scrutiny" for creative workers.
If I do have a huge deadline and have to double or triple that in a day I find myself mentally wiped and kind of useless for several days there after.