Melasma or Post-Inflammatory Hyperpigmentation (PIH): This condition is common in infants, especially after skin injuries or rashes, leading to hyperpigmentation that can worsen over time due to inflammation and skin irritation.

Erythema Toxicum Neonatorum (ETN): Although typically seen in the first few days of life, some cases can persist, leading to hyperpigmentation. However, it's less likely at 4 months.
Infantile Hemangiomas: These benign vascular tumors can cause hyperpigmentation, especially if they ulcerate or involute, leaving behind pigmented skin.
Café-au-Lait Macules: These benign skin marks can become more noticeable over time and may be perceived as worsening hyperpigmentation.

Congenital Melanocytosis (Mongolian Spots): While generally benign, these large, dark skin markings can be a sign of underlying conditions in some cases, such as neurocutaneous melanosis.
Neurofibromatosis Type 1 (NF1): Café-au-lait macules are a hallmark of NF1, and their presence, especially in large numbers or in specific patterns, warrants further investigation.
Adrenal Insufficiency (Addison's Disease): Hyperpigmentation can be a symptom of Addison's disease due to increased ACTH levels, which stimulate melanin production.

McCune-Albright Syndrome: Characterized by café-au-lait spots, polyostotic fibrous dysplasia, and various endocrine disorders, this syndrome is rare but should be considered in the differential diagnosis of hyperpigmentation with other systemic symptoms.
Carney Complex: A rare genetic disorder that can present with multiple skin myxomas, lentigines (especially on the face), and other systemic features like endocrine tumors.
Waardenburg Syndrome: A genetic disorder that affects the development of melanocytes, leading to patches of hypopigmentation or hyperpigmentation, among other symptoms like hearing loss and eye abnormalities.