I am a bit confused that the ultrasound showed a 4cm hypoechoic solid mass and several cysts. Then when I had the CT scan the results only showed 3 cysts. Is it possible that the CT scan did not pick up the mass or that the radiologist didn't read the images thoroughly?

I have to ask why you did not discuss this with the radiologist or your doctor... they both could actually see the scans, and give you the best answers... nobody here can do anything but make guesses.