I work 4 hours a day, 4 days a week, and I worked as a senior software engineer for Apple doing exactly this, successfully so. Most companies I've worked for (and I select them based on this) don't hold people to 8-hour work days and 5 day work weeks. Those kind of hours are ridiculous and everybody knows it; nobody (in my field) can be productive 8 hours a day.

Ever since I switched from 8h/day 5d/week to 4h/day 4d/week I'm far more productive than I've ever been. Hard focus for 4 hours is way more efficient than fucking around for 8 hours a day. In software engineering that's also where the worst bugs appear, out of annoyed/tired/overworked brains.

Now I work freelance and I let my clients know that I'll work my own hours, but I get shit done. So far, so good.