**Core Concept**
The fourth heart sound (S4) is a low-frequency sound heard during late diastole, corresponding to the contraction of the atria and filling of the ventricles. It is an important physiological phenomenon that can be related to various cardiac conditions.

**Why the Correct Answer is Right**
The correct answer is C. Heard during ventricular filling phase. The fourth heart sound occurs when the atria contract and fill the ventricles, specifically during the late diastolic phase. This is due to the increased pressure in the ventricles, which causes the atrioventricular valves to bulge back into the atria, producing the characteristic sound. The S4 sound is often associated with conditions that affect the ventricular compliance or the atrial contraction, such as hypertension, left ventricular hypertrophy, or cardiac amyloidosis.

**Why Each Wrong Option is Incorrect**
**Option A:** The S4 sound is typically not audible to the unaided ear due to its low frequency, which is usually below 20 Hz. It requires a stethoscope to be detected.
**Option B:** The frequency of the S4 sound is indeed less than 20 Hz, typically between 15-25 Hz. Therefore, option B is incorrect.
**Option D:** The S4 sound is not associated with the ventricular ejection phase, which corresponds to the contraction of the ventricles and the ejection of blood into the aorta and pulmonary artery. Instead, it occurs during the ventricular filling phase.

**Clinical Pearl / High-Yield Fact**
A key clinical correlation is that the S4 sound is often associated with conditions that cause left ventricular hypertrophy or decreased ventricular compliance, such as hypertension or cardiac amyloidosis.

**✓ Correct Answer: C. Heard during ventricular filling phase.**