Osgood-Schlatter Disease: This condition is a common cause of knee pain in adolescents, especially those involved in sports that include running and jumping. The pain is typically located just below the knee and is exacerbated by activity. The duration of symptoms (4 months) and the patient's age support this diagnosis.

Patellofemoral Pain Syndrome: This condition is characterized by pain around or behind the patella, often associated with activities that put stress on the knee, such as running and jumping. The patient's symptoms of knee pain with running and jumping make this a plausible diagnosis.
Medial Meniscus Tear or Strain: Meniscus injuries can occur from acute trauma or repetitive stress, leading to pain, especially with twisting or bending movements. Although less common in adolescents without a clear history of trauma, it remains a consideration given the patient's symptoms and activity level.
Tibial Tubercle Apophysitis ( Binding-Larsen-Johansson Disease): Similar to Osgood-Schlatter disease but affects the lower pole of the patella. It's less common but should be considered in the differential for knee pain in adolescents.

Osteosarcoma or Other Bone Tumors: Although rare, bone tumors can present with persistent pain that worsens over time. It's crucial to consider these in the differential diagnosis, especially if the pain is persistent, worsening, or atypical.
Septic Arthritis: An infection within the joint space, which is a medical emergency. Symptoms can include severe pain, swelling, redness, and warmth of the joint, along with systemic symptoms like fever.
Slipped Capital Femoral Epiphysis (SCFE): More common in obese adolescents, SCFE is a condition where the ball at the upper end of the femur (thigh bone) slips out of the socket. It can present with knee pain due to referred pain from the hip.

Chronic Recurrent Multifocal Osteomyelitis (CRMO): A rare condition characterized by recurring episodes of bone infection, which can cause pain and swelling. It's more common in children and adolescents.
Kohler's Disease: A rare condition that affects the tarsal navicular bone in the foot but can sometimes refer pain to the knee area, though this would be less likely given the specific location of pain described.
Juvenile Idiopathic Arthritis (JIA): While JIA can cause knee pain, it typically presents with additional symptoms such as morning stiffness, swelling, and potentially systemic symptoms like fever or rash.