Molluscum contagiosum: This is a common, benign skin infection caused by a poxvirus, characterized by small, pearly, or flesh-colored bumps. It's highly contagious and often seen in children, making it a likely diagnosis for a skin lesion in a 4-year-old.

Wart (Verruca): Caused by the human papillomavirus (HPV), warts are common in children and can appear anywhere on the body. They are usually harmless but can be bothersome.
Eczema (Atopic Dermatitis): A chronic skin condition characterized by dry, itchy, and inflamed skin. It's common in children and can cause a variety of skin lesions.
Impetigo: A highly contagious bacterial skin infection that can cause red sores, blisters, or crusts on the skin. It's common in children and can spread quickly.

Meningococcemia: Although rare, this condition is life-threatening and can cause skin lesions (petechiae or purpura) as part of its presentation. It's a bacterial infection that can lead to meningitis or sepsis.
Childhood Leukemia: Certain types of leukemia can cause skin lesions, although this is less common. Given the potential severity, it's crucial not to miss this diagnosis.
Neurofibromatosis: A genetic disorder that can cause skin changes, including café-au-lait spots and neurofibromas. While not immediately life-threatening, early diagnosis is important for management.

Juvenile Xanthogranuloma: A rare, benign skin condition that can cause yellowish lesions, often on the face, neck, or extremities.
Langerhans Cell Histiocytosis: A rare disorder in which the body accumulates too many immature Langerhans cells, leading to a variety of symptoms, including skin lesions.
Basal Cell Carcinoma or Other Skin Cancers: Extremely rare in young children but possible, especially with certain genetic conditions or significant sun exposure.