TOKYO/SAN FRANCISCO (Reuters) - Japan's Toshiba Corp (>> Toshiba Corp) has narrowed down the field of bidders for its chip unit to four suitors including Broadcom Ltd (>> Broadcom Ltd) and Western Digital Corp (>> Western Digital Corp), two sources with knowledge of the matter said.

South Korea's SK Hynix Inc (>> SK Hynix Inc) and Taiwan's Foxconn (>> Hon Hai Precision Industry Co., Ltd.) are the other bidders, they said, adding that Broadcom has partnered with U.S. private equity firm Silver Lake Partners LP.

The sources declined to be identified as they were not authorised to speak on the matter publicly.

There were about 10 bidders in the first round of offers, sources have said previously.

Toshiba has said it needs to sell most or all of the prized business to cover charges related to U.S. nuclear unit Westinghouse Electric that threaten the Japanese conglomerate's future.

(Reporting by Taro Fuse in TOKYO and Liana B. Baker in SAN FRANCISCO; Writing by Tim Kelly; Editing by Edwina Gibbs)

By Taro Fuse and Liana B. Baker