I've had fair success with similar arrangements (4 days instead of 5) and have found that I am just as productive in the 4 days as I am in the standard 5. The reduction in stress and the enjoyment that comes from pursuing your own interests has a tremendously positive impact on both your life and your work.

Arranging part-time conditions can be a slightly bumpy process: as stated above, management is often initially offended that you don't want to dedicate your life to their company, but framing your request correctly can smooth over most bumps.

When requesting a part-time arrangement I usually present it quite honestly: referring to a need for work/life balance, but also referring to the learning outcomes from pursuing challenging projects and the positive impacts they will have on my experience and capacity at work.

At the end of the day most managers realise that they are faced with either agreeing or facing the inevitable lose of an employee, but the decision can be made easier for them if you highlight the positive outcomes for your performance at work.

Incidentally, when I've described my part-time work schedule to friends and family, the universal response has been amazement and jealousy. The desire for a better work/life balance is such a common thing that I'm curious as to why so few people pursue better arrangements...?