How do you solve a problem like replacing Microsoft's outgoing CEO? Trimming the fat from a long list of potential candidates is one option, and it looks like the company has done just that.

According to Reuters' all-knowing anonymous sources, the software and services giant has whittled down a list of 40 potential candidates to leave at least four remaining.

They include ex-Nokia chief Steven Elop, former Skype CEO Tony Bates (who now runs Microsoft's business development), Ford CEO Alan Mulally and Satya Nadella, who oversees Microsoft's cloud and enterprise services unit.

On rumour

All four names have been passed through the rumour mill in recent times - Elop has long been backed as Ballmer's potential successor due to Microsoft and Nokia's special relationship.

Ford's Mulally has repeatedly made clear his desire to stay at the motor company's helm until 2014, though he may be persuaded to pursue other glittering opportunities should they arise, according to Reuters' sources.

Names not to appear in the list include ex-Windows chief Steven Sinofsky (who appeared in our own list of six potential candidates) and former Juniper Networks CEO Kevin Johnson.

Microsoft has until August 2014 to find a replacement for Ballmer after the larger-than-life exec announced his departure from the company in a blaze of teary, Dirty Dancing-styled glory.