I too am looking to work less than the standard 40 hours a week for similar reasons. Only committing to 4-6 hours in a given day also gives me more freedom to take a large break in the middle of the day to go for a walk or a run, which can be useful in the Scandinavian winter when the sun is only up for a relatively limited time each day.

There are some political parties in Norway and Sweden who want to make a six hour work day the norm.

There are many good arguments for significantly reducing the amount of time the average person works, both for the benefit of individuals and community: http://www.neweconomics.org/publications/entry/21-hours