Diagnosed with lesion between l4 & l5. no pain or symptoms. discovered during mri for other diagnosis. operate?

No: To entertain surgical intervention for presumed asymptomatic disc disease is foolish. Disc surgery caries a thirty percent chance of worsening condition. If intractable pain or muscle weakness then time to consider. Not before.

Depends: On what the "lesion" is. This is a very vague term. If it is a synovial cyst without symptoms, then, no. If it is a tumor, then yes. Need a lot more info. If they can't give you a better answer as to what it is, you can send images here with pvt message or get second opinion somewhere.