On our small med-surg floor we work 8's and 12's. A lot of the newer nurses (including me) would prefer to work 3 12's but there are enough older, experienced nurses that only work 8's that it would be impossible to go to all 12's. Plus enough nurses that want 40 hours instead of 36.
Anyway, our floor does have 3 part-time positions that are 20-24 hours, usually 3 8's or a 12 and a 8. If a full-time person needs someone to work for them we are supposed to ask a part-time person first before a per diem person, so sometimes they end up working more than 24 hours if they want to.
As a side note - I hate 4 hour shifts, or patient assignments that only last 4 hours. For example, I worked 3-11 the other night and from 3-7 had a patient assignment that the 7-3 day shift nurse had had, then from 7-11 floated so the 7p night shift nurses coming in could have their assignments for the night. Makes sense, but having a patient assignment for only 4 hours barely gives me anytime do my patient care, get my charting done, and have enough time to know enough about my patients to give a good report to night shift.