The 2010 murder of Sosilawati Lawiya, 47, her driver, lawyer and bank officer shocked Malaysians and dominated the headlines for weeks.
The four were reported missing after going to discuss a land deal with the convicted lawyer and his brother on their farm near the sleepy town of Tanjung Sepat, 80 kilometres (50 miles) west of the capital Kuala Lumpur.
Police said the victims' bodies were torched and the remains scattered in a stream.
"The prosecution has proven its case beyond a reasonable doubt," Judge Akhtar Tahir was quoted by The Star online as saying.
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The four convicted were lawyer N. Pathmanabhan, 43, and three farm hands aged between 21 and 33.
Initially both Pathmanabhan and his bother were detained, but only Pathmanabhan was charged over the murders. The other brother was released.
The four can appeal the verdict but their lawyers could not immediately be reached.
Pathmanabhan was accused of masterminding the killings in a land deal gone bad. Sosilawati, a self-made millionaire, founded Malaysia's popular Nouvelles Visages line of cosmetics.
Crime is perceived to be on the rise in Malaysia, and the government has said it will address Malaysians' increasing fears.
Police say statistics show they have managed to reduce crime, but anecdotes of break-ins, snatching and violent crimes abound in papers and on social media.