As a salaried non-exempt employee, if I get my work done in less than 40 hours in a particular week, can I just leave and still be paid for 40 hours?

No. The expectation is that if you are regularly scheduled for 40 hours in a workweek, that you will have enough work to do within those 40 hours. If you complete your work in fewer than 40 hours during a workweek, then you should notify your supervisor. Your timesheet must account for 40 hours per workweek.