The 40 hours split into 5 days is so ingrained into American culture that anything different wouldn't be seen as normal. I had a job I loved that we did 40 hours in 4 days. It was nice having that extra weekday off.
4/10's are fantastic, especially if you commute. The extra 2 hours a day really doesn't feel like much, and I often find I have better "productive" time than in a standard 8 hour day.