That depends. They can take anybody off road, but they can't remove them from the payroll. You are still guaranteed your 8.
And it can get complicated past that. If you already worked 5 days, Mon-Fri, then you already got your weekly guarantee and they do not have to work you Saturday.
If you are Tue-Sat and have only worked 4 days, then they still owe you 8 for Saturday. They have to find some non-driving work for you to do.
Now, here's where it gets a little complicated. If they need drivers over and above the Tue-Sat drivers, it becomes extra work. They can work anybody who has not gotten their 5 days in first, but after that, it goes by seniority.
So, if you want to work a 6th day and your seniority allows you to work, they still have to find you 8 hours of non-driving work to do.