Good question. I am a software developer in Germany and on every interview now and then I ask for the 4-day-week. It was denied 11 times so far. Most employers consider this to be a joke and cannot believe I am serious.
After half a year in a company you have a right to get into part time work, but there are some requirements that have to be met and every employer I talked to in my career so far was very... unhappy when asked about it.