The big issue I have with 4 day weeks is for it to work well you need it to be well co-ordinated in terms of who is off when. If you work a 4 day week and everyone you interact with is off the same day (like at say Xmas/Easter) that's great. Very little in the way of comms from colleagues, customers etc (in an office job). The issue is say I have Monday off but others who generate work for me do not. I come back on Tuesday with a backlog of work to greet me, emails to read etc. On the flipside you might need something from someone else on the Tuesday, but they are off. This obviously happens today due to absences but it would be much more prevalent with 4 day weeks I think.
What's worse is when there is an imbalance e.g. I used to work with someone who did a 3.5 day week (reduced hours) whilst others did 5. The problem was, there was sort of an inherent expectation from some that she'd deliver as much as people doing a 5 day week.