But then you need to hire more staff. I think part of the sell is that it's no more cost for the employer, since workers get more done in less time. That might not be true for many operational jobs.

Also to your other part about "it's not that hard"

It actually is. We cannot get enough bodies in the mill to not work everyone at full time. We pay 18 year olds 6 figures to operate a mill and we still cannot get enough bodies to come anywhere close to working a 4 day week.

It straight up doesn't work that simply when you're running enormous 24/7 operations in critical industry. Thats like all the football fans on the couch yelling at the coach "JUST DO X". Really easy to say "do X" but the application becomes extremely difficult. Yeah in an office, sure....in a refinery where you create base stock products that allow hundreds of other major plants to run to produce all the basic products you use every single day? Not gonna work that way

This isn't some machinist shop that takes orders. it's a multi billion dollar full rip steel mill/refinery/plant/etc. that loses LOTS of money when it's not at full capacity. That has lasting knock on effects on other industries for example when base manufacturing can't keep up

You have salary workers like process engineers for example. Working less than 5 days regularly just isn't acceptable. I would not be effective in my job if I only had 4 days a week to get everything done. Also someone always has weekend coverage on "off" days for salary or holidays. So you're still "working"

Also many plants have minimum hour requirements in their union contracts where we have to run X days minimum a week or we still pay. There is more to the puzzle than just the office sector.

You have shifts and shift workers yes but again, the mill basically needs to run 24/7. so lots of people get forced for OT, or willingly take it

Edit: I love the downvotes with no refutement. I am not talking from no experience here. I actively work as a process engineer in a steel mill and actively deal with these problems DAILY. Moving to a 4 day week changes nothing in 24/7 operations. You have to run all of the time, end of sentence (or your mill is getting shut down, and you all lose your jobs...nobody in the mill wins there). The compensation is through the roof and most people end up pulling in tons of overtime. I dont know many other jobs that an 18 year old can pull in an EASY 6 figures with no form of education past high school. The hourly guys make WAY more than any of us salary folk (me and other engineers have spoken candidly with guys on the floor, and they pull in well over 100,000 with no overtime), and on top of that, there are guys who get legitimately pissed when they can't get enough overtime or work more hours cuz they want that money

hiring something like 30% more people.

I bet it was ( 5 / 4 or ) 25% more.

Having that one day when external coordination is impossible is why absolutely no business is ever open from Sunday to Thursday or Tuesday to Saturday since they all failed. Except they are.

If you need some scheduling help, you may want to buy a computer and have it help the crunching. At the famous fast food restaurant a while ago, we used tools to ensure adequate coverage against workload; and your problem sounds the same except with one fewer variable. It was 35 years ago, so it probably works as an app on your watch, now.