Toshiba has selected four bidders — private equity firms Bain Capital, CVC Capital Partners and Brookfield Asset Management and a Japanese consortium — to proceed to a second bidding round, people briefed on the matter said.

The race for what could be the country's biggest buyout deal this year is gathering pace, as the four bidders are now invited to conduct due diligence for several months.

The consortium involves state-backed Japan Investment Corp. (JIC) and domestic private equity firm Japan Industrial Partners (JIP), said the sources, who declined to be identified as the matter is private.

One of the sources said Brookfield's proposal is for a capital alliance that would see the Japanese conglomerate remain listed, while the other three propose taking it private.

Bain and Brookfield declined to comment. CVC, JIC and JIP could not be immediately reached for comment. Toshiba said it would not comment on details of the bidders.

The bidders are likely to tie-up with others, potentially those who did not make the second round, separate sources said.

KKR & Co., Apollo Global Management, Blackstone, Baring Private Equity Asia and MBK Partners had all expressed initial interest in Toshiba.

KKR decided not to following through with a bid at this point, sources have said, as it was awaiting more clarity from the government and Toshiba's management on whether a deal could realistically materialize.

One of the bidders in the first round was considering offering up to ¥7,000 ($50.67) per share, valuing the deal at about $22 billion.

But that bidder failed to go into the second round, one of the sources said.

Toshiba said Tuesday it had selected multiple bidders to proceed to due diligence without naming them. The company last month received eight initial buyout proposals as well as two offers for capital alliances.