-- Updated with stock move and potential Yahoo! breakup strategy.NEW YORK ( TheStreet) -- With Carol Bartz out at Yahoo! ( YHOO), the list of CEOs on the brink shrinks to four. Tumultuous markets, slumping sales and stalled turnaround strategies put a lot of pressure on CEOs. And as Bartz ouster showed, it's only a matter of time until the board decides to shake things up.