OMAHA, Neb. — Berkshire Hathaway revealed four new investments Thursday in steelmaker Nucor, insurer UnitedHealth and two of the nation's biggest homebuilders — Lennar and DR Horton — but it's not clear if the investments are big enough to ensure that legendary investor Warren Buffett handled them.

Buffett, who plans to retire as CEO at the end of the year after six decades of building Berkshire, handles all of the conglomerate's biggest investments worth $1 billion or more. Three of the new investments disclosed Thursday in a filing with the Securities and Exchange Commission are worth less than that, so they could be the ideas of one of Berkshire's two other investment managers.

But Berkshire’s new UnitedHealth stake that was also revealed Thursday was worth $1.57 billion, so tha

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