If you reserve with the Amex Platinum CDP you can see the 4-hour grace period allowance right on Hertz's website. For example, if you use any random CDP and book a rental for 1 day and 4 hours, you will get charged for 2 days. If, however, you then change the CDP to the Amex Platinum CDP, the same duration will only reflect a 1-day rental due to the Amex Platinum 4-hour grace period.

I would assume if you have a corporate CDP that has a similar benefit, you would see the same thing in a test booking. If not, then you only get the standard 29 minute grace period.