Activist investor Carl Icahn said he bought another half-billion dollars' worth of Apple stock on Tuesday, boosting his stake to more than $4 billion. Last Wednesday, Icahn had disclosed on Twitter another $500 million purchase that took him to a $3 billion position. He did so again on Thursday.

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."

Icahn is urging Apple to return more cash to shareholders, and has filed a resolution proposing that it gives back $50 billion more through share buybacks.

Apple said on Monday it had returned an additional $7.7 billion in cash to shareholders through dividends and share repurchases during the December quarter.