After David sustained a traumatic brain injury, he has discovered that he can't remember anything that happened in the last four years, but he can learn new information just as well as he did before the injury. David is likely experiencing amnesia.

Retrograde amnesia

Explanation:

Amnesia. can be defined as a form of memory loss. usually people with amnesia find it difficult forming new memories. there are four types of amnesia

Retrograde amnesia: when an individual is hit with retrograde amnesia, such a fellow will lose existing and previously made memories. This type of amnesia tends to affect recently made memories,  older memory are usually affected slowly. that is why David discovered that he can't remember what happened to him in the last four hours after he sustained a traumatic brain injury.