(Reuters) - Carl Icahn said on Tuesday he bought another half-billion dollars' worth of Apple Inc stock, raising the activist investor's 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 Apple's buyback program, but hope they win that race."

(Reporting by Edwin Chan in San Francisco; Editing by Jeffrey Benkoe)