I find it odd to limit ourselves to the structure of 4-day workweek or 40hr work-week
I utilize all 7 days to work on my professional & personal projects
To do everything sustain-ably and optimize output, for some projects, I dedicate couple of hrs for 4 days and for some only a day and for some everyday.
How you time-box your work for maximum potential is a personal discovery and the strategies vary based on the person, environment and the nature of the project.
So if we are talking about weeks, I have 7 days to get things done which are important in my life and I will neither follow 40hr work-week not 4-days a week. I will see the project, where it stands against all things in life, put it somewhere in my 7 day schedule, see if this schedule works and go ahead with the learnings each week.