Often not advertised explicitly, but try just asking :). If you have the skills an employer wants, and you aren't demanding the max pay possible, I've found (for myself and colleagues) the barrier to this happening is mainly people having the nerve to ask, or to take a salary hit of say 10-15% net, rather than the employer being disgusted and saying no. In fact, a lot of managers quite fancy doing a 4-day week themselves, and even if they're not to the point of doing that they may actively allow a subordinate to do it as it sets a precedent which they may feel makes it easier for them to ask for the same one day.
