In that case, you should be fine since your original reservation was for 4 days even and your new reservation is the same. You shouldn't have any issue.

Keep in mind though that the reservation system will still assume you are returning at 7PM, so the estimate on your rental contract could be much higher than your reservation amount. Don't panic. Just return on day 4 by at or before the same time you picked up on day 1 and everything will adjust back to your original rate when you return the rental.