Yes, the border between a daily rental and a weekly rental is typically 5 days. As to your question of when the 5th day kicks in, it's typically 4 days and some number of hours. It can vary by company, but it's typically around 3 hours.

I just priced out a rental at LAX for 4 days and 2 hours. Here were my results:

Hertz - 4 days and 2 hours
Dollar - 4 days and 2 hours
Thrifty - 4 days and 2 hours
Avis - 5 days
Budget - 5 days
Payless - 4 days and 2 hours
Enterprise - 4 days and 2 hours
National - Weekly Pricing
Alamo - 4 days and 2 hours

Then I priced out the same rental for 4 days and 3 hours:

Hertz - Weekly Pricing
Dollar - Weekly Pricing
Thrifty - Weekly Pricing
Avis - Weekly Pricing
Budget - Weekly Pricing
Payless - Weekly Pricing
Enterprise - Weekly Pricing
National - Weekly Pricing
Alamo - Weekly Pricing

As you can see, different company handle things slightly differently, and it may also be case specific depending not only on company but location as well. Typically though the threshold is 4 days and 3 hours when weekly pricing kicks in.