Your 4 year old will not need ID (she's with you), but you definitely do. You will be asked for it at least once and probably more than once before you actually board the plane.
An e-ticket just means there is no paper ticket to lose or have stolen. The airline has a record of your ticket in its computer. Usually, all you need at the check in desk is to know what flight you have the ticket on and your ID. I do carry the credit card used to purchase it (may need it on the trip anyway) and as nluvwithdisney said, it may be required to use the special check in terminals at the airport. These special terminals often have no lines and are usually much quicker than standing in line waiting for an agent. To use them, you just follow the directions on the screen. There are generally airline employees nearby to assist if you need help.
In addition to your ID, you should also bring a copy of your itinerary or your e-ticket receipt (your computer printouts). Some airports require pasengers to have one of those, a paper ticket or a boarding pass to get through security.
Have a good trip!
