Normal Variant: The mild diffuse bladder thickening of 4 mm may be due to incomplete distention, as mentioned. This is a common finding in ultrasound scans, especially if the bladder is not fully distended.

Urinary Tract Infection (UTI): Bladder thickening can be a sign of a UTI, which is a common condition in adolescents. Symptoms such as dysuria, frequency, and urgency may accompany this finding.
Overactive Bladder: This condition can cause the bladder wall to thicken due to increased muscle tone. Symptoms include urinary urgency, frequency, and incontinence.
Neurogenic Bladder: Although less common, neurogenic bladder due to a neurological condition can cause bladder wall thickening. This would be more likely if there are underlying neurological symptoms or conditions.

Bladder Cancer: Although extremely rare in a 14-year-old, bladder cancer can cause bladder wall thickening. It's crucial to rule out this possibility, especially if there are other symptoms like hematuria.
Tuberculosis of the Urinary Tract: This infection can cause bladder wall thickening and is a critical diagnosis not to miss due to its potential for severe consequences if left untreated.

Schistosomiasis: In endemic areas, schistosomiasis can cause bladder wall thickening and other urinary tract symptoms. This diagnosis would be considered rare in most parts of the world but is crucial in specific geographic locations.
Bladder Endometriosis: Although rare, endometriosis can affect the bladder and cause wall thickening, typically in females but could be considered in cases where the diagnosis is unclear.
Eosinophilic Cystitis: A rare condition characterized by eosinophilic infiltration of the bladder wall, leading to thickening. It can be associated with allergies, infections, or other conditions.