It depends where you fall in tax bracketing a bit too ..The four day working week idea isn’t doing the same hours over less days. Any employer can do that and the government would say nothing. It’s working 20% less hours or 28 hours instead of 35.
I was lucky I guess , worked in healthcare, dropped from 35 hrs over 5 days to 30 over four days .
Worked out about 5quid an hour worse off , so did my self employed thing on my day off and earned treble what that extra day was