My wife's employer decided to change how vacation is paid they were getting 5 days of 8 hours for a total of 40 hours per week. Currently they are working 12 hour shifts of 4 days one week 3 the next. They stated they will only pay 3 days of 12 hours for 36 hours for a week of vacation this is 4 hours less then a normal full work week and doesn't take into accout the week they work 4 days. Can they pay you less then 40 hours for a vaction week?

They are not required to provide paid vacation at all, so yes, they can.

No federal or state law requires vacation at all, so employers can make up their own rules on how to award it.

The employees were working 40 hours a week and getting some amount of vacation based on that (the amount was not stated, but I will assume 40 hours a year).

So, when the employees are working 48 hours one week and 36 hours the next, how much vacation are they getting over a year?

In the first instance, 2080 hours of work and 40 hours of vacation.

IF the company pays vacation based upon hours worked, the 48/36 schedule will pay 42 hours of vacation per year.

But, there is no law requiring that they do that or that they pay any vacation at all.