I am working 75% since about 7 years now. However, I did not chose 4 days, but 6 hours (and 5 days). I like this much more because I can more evenly distribute my workload and I have a more regular schedule, including recovering from work on a daily basis. 6 hours is the maximum I found I can concentrate and be productive. Afterwards, my output always decreased significantly and I was mainly wasting time.