(Reuters) – Carl Icahn said on Tuesday he bought another half-billion dollars worth of Apple Inc stock, raising the activist investors stake to roughly over $4 billion. Icahn said the stock is a no-brainer, and argued the shares were undervalued. He is trying to get the company to tack on an additional $50 billion in share buybacks. On Tuesday, he tweeted: Just bought $500 million more $AAPL shares. My buying seems to be going neck-and-neck with Apples buyback program, but hope they win that race. (Reporting by Edwin Chan in San Francisco; Editing by Jeffrey Benkoe)