I know someone who works there.
Yes it is an average four day week. Here's one of the links at Euston from a while ago, so will give a feel for it. There's four links there apparently, so quite a large depot. At Euston some weeks are 4 days and some 5, so the week of carried over rest days from the 5 day weeks give a week of rest days about every six to seven weeks.
I understand that other depots (apart from Polmadie mentioned above) are all 5 day weeks with a week of rest days every fifth week.
Yellow is 'weekly spare'. Can be given any other week to cover and get given the rest day* for that week - so the rest day on weekly spare is effectively meaningless as it will be moved.
*If you get given a four day week off weekly spare, you decide which rest day you are keeping and which day you'll be working 'daily spare' (presumably with a datum time of a surrounding day - with unlimited movement), as all the weekly spare weeks are five day weeks.
Blue is 'daily spare'. Unlimited movement, but can't finish later than 11 hours after datum time as mentioned in another post above.
Green is 'route refresh' or 'safety brief'.
Red is 'Bi-Di brief.
Rest days worked have a booking on allowance, I think it's £75 but I'm not 100% on that + flat rate hours worked.
Bank Holidays are treated as normal working days.
I think Sundays are flat rate, but can't remember if there was a booking on allowance similar to rest days worked or not.
There is an agreement that they can't do 'double trips' - so Birmingham and back twice can't be done. They either do an out and back; or go to Birmingham, pass to Preston, Preston to London in a sort of triangle.
The garbage terms and conditions, with a huge amount of being messed about with days off changed and massive movement of times at less than a week's notice, are what the rest of us are fighting not to have forced on us, but they have already been in from Virgin days.
Everything that's been sold, is an aspiration to 'get in' for other TOCs with less desirable work.