If you pay any tax @ 40% then it will cost you less than 20% of your nett pay to go 4 days. I would want to calculate that myself. Also I want to work out exactly how it might affect pension/AVC. You should be entitled to pay for Bank Holiday Monday's or a day off in lieu. Methinks that you'll do enough hours to ensure that you are entitled to min. 20 days annual leave. Unless it's stressfully busy I wouldn't be too concerned about extra work over the 4 days, days might zip along. Personally I like the idea of a Wednesday off to break up the working week and simplify things re public holidays.