There is probably at least a few employees who can do a full day of productive work in 4-5 hours. Forcing them to sit out 8 hours, even on flexible basis, is very discouraging. Establish a reasonable goal per week, and have them do that in however many hours it takes.
I agree. I have 4-5 hours of productivity myself, and the rest of my time is just spent pretending to work or reading tech sites because that's how things go; management doesn't understand the amount of mental burnout that happens in IT.
If you need me to do something an hour before I'm supposed to go home, more often than not I will fuck things up severely. I'm done, my brain is pudding. I will make mistakes that could be published on TheDailyWTF. I will spend that hour doing something that I'd be able to do properly in five minutes next morning.
Still, flexible working hours are one step towards changing things around. I worked 9-5 at my last job for a while, then I slowly got them to let me eat at the office and go home early, then work flexible hours, then I got them to let me work from home occasionally, and finally, they just gave me "things to do this week" and I was free to do those things however I wanted to. That was especially nice during the winter, when days are short; I'd spend mornings and early afternoons walking around, cooking, etc, then working from home in the evening, being more productive than ever.
I quit because management changed. While they were willing to let me continue working on my own terms, I realized their business ideas were a colossal fuckup and I was on a sinking boat. Now I'm negotiating flexible working hours again and I don't doubt I'll be where I was in a year, maybe two. (The boat sank, by the way.)