Only works if everybody agrees on which day is the '5th' day. My wife has chose to work 4 days a week (80%) and she has to constantly fight to keep her free day actually free, since people keep trying contact her or book stuff with her on that day.

We tried this several times, specifically banning meetings on Friday, and it never sticks. Some manager or executive always winds up ignoring the rule and setting a meeting anyway, which throws the whole thing out.

We try to nudge towards making this possible by having a "soft rule" discouraging scheduling any standing or cross-team meetings on Fridays.