Viral Infection: This is the most likely diagnosis given the elevated lymphocyte count, which is a common response to viral infections. Many viral infections, such as upper respiratory tract infections, can cause an increase in lymphocytes.

Bacterial Infection: Although less likely than viral infections to cause a significant increase in lymphocytes, certain bacterial infections can lead to an elevated lymphocyte count, especially if the body is mounting a strong immune response.
Allergic Reaction: Allergic reactions can sometimes cause an increase in lymphocytes as part of the body's immune response.
Vaccination Response: Recent vaccination can stimulate the immune system, leading to an increase in lymphocytes.

Leukemia/Lymphoma: Although rare, it is crucial not to miss a diagnosis of leukemia or lymphoma, as these conditions can present with abnormal blood cell counts, including elevated lymphocytes and monocytes. Early detection is vital for effective treatment.
Infectious Mononucleosis: Caused by Epstein-Barr virus, this condition can lead to significant increases in lymphocytes and sometimes monocytes, and can be serious if not properly managed.
Tuberculosis: In some cases, TB can cause an increase in lymphocytes and monocytes. Given its potential severity and the importance of early treatment, it should not be overlooked.

Lymphoproliferative Disorders: These are rare conditions characterized by an excessive proliferation of lymphocytes, which can be benign or malignant.
Monocytic Leukemia: A rare form of leukemia that affects monocytes.
Chronic Inflammatory Conditions: Certain chronic inflammatory conditions can lead to elevated lymphocyte and monocyte counts, though this would be less common in a 4-year-old without a known history of such conditions.
Immunodeficiency: Some immunodeficiency disorders can present with abnormal lymphocyte counts, though typically these would be decreases rather than increases.