[quote littlepattilou]@MrsCremuel YANBU at ALL.
It's great to do 4 days a week, if that's what you started out as (3 is better IMO, but hey ho.)
But if you drop from 5 to 4, you will almost ALWAYS end up cramming the 5 days work into the 4 days. (as a few posters have said...) But with 20% less pay!
Happened to me some years ago. I dropped from 5, to 3.5 days a week. (4 days one week/3 the next,) but I still had the same workload!
I raised the issue a few times, but it fell on deaf ears. It got so bad I even had to take work home to catch up. 
The ones saying '4 days is fab' have either not gone from 5 to 4, OR they work in a job, like on a checkout or in a shop or cafe, or restaurant or something, where they don't have an allocated workload, like you do in some sectors...
If I were you, I would be thinking about looking for a new job that has the hours you want immediately. So the workload will suit the hours IYSWIM.[/quote]
How bloody insulting!
I’m in senior manager in HE, I have worked 5 days since I was 21, dropped to 4 days recently at 42 after 15 months leave having DD.
As part of my flexible working request I demonstrated what I would and wouldn’t do out of my previous role and a proportion of tasks have been reallocated elsewhere. I am never contacted on my non working day (Monday) and I don’t look at emails etc either. My PA ensures urgent tasks are brought to my attention first thing Tuesday.
You can be a successful, postgrad qualified professional and work effectively part time but it does depend on sector and on putting boundaries in place. Fortunately I am further along in my career and have some clout. But I also support more junior colleagues to achieve the same balance. About half of faculty have some flexibility in their hours.