I agree with your assessment. I am on call for a week once every few months. Sometime it’s light and still get work done other time all we do is field calls and put out fires. Normally we work 40 hours, sometimes more sometimes less. Saying I was only going to work 32 hours and not be available 1 day a week would not work very well. I suspect the employer would win by paying you less but the employee would end up working the same, sometimes more sometimes less. At the end of the day the work needs to be done no matter how long it takes. But I also work 2 jobs full time analyst / programmer and TA CS courses so I might just be an outlier. I make enough to only work 1 job, but I like helping students. The extra money does help and I would not do it for free. So short answer is NO I would not work 4 days a week for 80% pay.