There's not enough information in your post to answer definitively. When does your rental period end? And by "extend it", do you mean "return it 4 hours later than planned" or "pick it up 4 hours earlier than booked"?

Assuming you want to return it later, it depends on whether or not you are triggering another billing increment. For example, if your return date is Dec 31 at 10am and you want to keep it until 2pm, you will mostly likely incur an Extra Day charge as others have indicated. However, if your return time is 6am and you want to keep it until 10am, you will most likely incur no extra charge as you are probably already paying for a full day. The same applies if you are already paying a weekly rate but only keeping the car for (say) 6 days.

The actual details depend on your current reservation (how long it is and what the breakpoints are on daily/weekly/monthly charges).