The answer probably depends on the specific week coupons that you're looking at, but
my guess would be that if you didn't keep the rent a car for the full five days, the coupon would be invalid.
You say that you are renting "just short of 5 days"... as long as you're renting a total of at least 99 hours from pickup to return, your rate would be counted as 5 days.
What you would want to look at are the terms and conditions listed on the coupon, specifically with regards to a minimum and maximum rental length. Generally, weekly car rental coupons list a minimum requirement of 5 days-- and the 5 day requirement is for the rental, not just the reservation. Often weekly car rental coupons also require that you keep the rental car over a Saturday night, as well.
This is why, when you are getting price quotes, often the value of a rental car coupon is not caluculated in your price quote.
This is because the rental provider determines at the END of the rental (when you return the vehicle) whether or not you met all the terms and conditions of the promotional offer that you used.
If you meet the conditions of the week coupon that you have, you should be able to use it.