There is some debate in the medical community as to whether fourth-degree burns really exist, or if a three-tiered classification system is sufficient. Most, however, use the term "fourth-degree" to describe injury that is irreparable, usually due to electrical burns. Generally speaking there is permanent damage to the muscle tissue, tendons and bone (although a more strict definition extends to bone and cartilage damage only).
Fourth-degree (or transmural) burns will definitely require hospitalization, and may require amputation of extremities or other surgical interventions. In some cases, fourth-degree burns can be fatal, so it is of upmost importance that the burn victim be rushed by ambulance or EMT to a burn unit or medical treatment facility immediately.
While waiting for the paramedics to arrive, determine whether or not the person is breathing and, if not, gently attempt CPR. If they are breathing and are able to drink, have water available. They've lost a lot of moisture and are probably dehydrated, not to mention that their body will use the water to repair/recover itself. Talk to them. While there's not much you can do in the way of making them comfortable, keeping them conscious will improve their chances once assistance arrives.
Information for this node was provided to me by a nurse at the hospital but may also be found in part on the web at http://www.burnsurvivorsonline.com/injuries/degree.asp