Where I work we do a mix... Some 4 days, 10 hrs each, some 5days 8 hrs each.
Production is typically 4/10 and most other people are 5/8. This allows the other groups a day where production isn't running and other things can happen, like maintaining equipment and running tests without interfering with production schedules. While not requiring support to come in on a Saturday or Sunday.
It works out pretty well for us, except when production does ot on Fridays for weeks in a row and the other stuff can't get done...