You are eating way too much tuna and are at risk for mercury poisoning. If you weighed 190 pounds, at 4 cans per day, you would be greater than 0.9 mcg/kg/dy of mercury, which is way above the 0.1 mcg/kg/dy that's considered the safety limit.
One of the web calculators said you were 4.6 times the safety limit for mercury toxicity.
http://www.gotmercury.org/
I would choose other forms of protein and actually advise you to see a physician to get tested for mercury toxicity. Your risk of cancer and other serious health disorders are significantly increased because of your tuna consumption.
Gotta love how we've poisoned the environment to make a benign fish into a toxic substance.