all of us immediately agreed that we cannot get the work done required of us + do the extras of being a floor process engineer in only 4 days.

1. the plural of 'anecdote' isn't 'data'.
2. most/all 4-day-work-week experiments collected statistical data showing a strong trend suggesting the extra rest time made both the rote/repetitive work cleaner and the knowledge/expert work faster.

Unless your shop is a weeeird outlier, you stand every chance of coming out of a 4dww trial working more efficiently, more rested, more reliably, more effectively. That's actually a very common theme in all this, that any 'loss' in time or money is eaten up by better work done more effectively when we're not so tired we're microsleeping all day and wondering where we are and where we were.