**Core Concept**
Class 4 cavities are a type of carious lesion that involves the proximal surfaces of molars and premolars, extending to the interproximal area between two adjacent teeth. These lesions can be challenging to diagnose and treat due to their complex anatomy and potential for pulp involvement.

**Why the Correct Answer is Right**
The correct answer to the number of line angles in a class 4 cavity is 5. This is because class 4 cavities typically involve the proximal surfaces of two adjacent teeth, which creates a total of 5 line angles. These line angles are the points where the proximal surfaces of the two teeth meet, and they are critical areas for caries progression.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it underestimates the complexity of the anatomy involved in a class 4 cavity. The number of line angles in this type of lesion is indeed higher than what is suggested by this option.
* **Option B:** This option is incorrect because it suggests a number of line angles that is too low for a class 4 cavity. The anatomy of these lesions is typically more complex than what is implied by this option.
* **Option C:** This option is incorrect because it suggests a number of line angles that is too high for a class 4 cavity. While these lesions can be complex, the number of line angles is typically not as high as what is suggested by this option.

**Clinical Pearl / High-Yield Fact**
When diagnosing and treating class 4 cavities, it's essential to consider the unique anatomy of these lesions and the potential for pulp involvement. A thorough understanding of the number of line angles and the corresponding anatomy is critical for effective treatment planning.

**Correct Answer: D. 5**