4cm mass on left kidney .what do that mean?

Mass on kidney: Not to sound silly, but that means there is a 4cm mass on your left kidney. Without any further information, that is something that would definitely need to be evaluated with more testing. Best wishes.

Kidney mass: First, it's important to know if it's a cyst (filled with fluid) or a solid mass (tissue). An ultrasound will be helpful to determine the difference. Cysts are very common and are usually found incidentally, and often don't require any intervention other than routine surveillance. A mass could be serious, and would require further testing. Your doctor can help you with the next steps of analysis.