The judge also ordered the seizure of millions of dollars and Swiss francs from the pair, who acted as go-betweens in the attribution of Nigerian oil exploration rights worth hundreds of millions of dollars.
Nigerian Emeka Obi and Italian Gianluca Di Nardo had requested an accelerated trial, which took place behind closed doors and allowed for lighter penalties if they were found guilty.
The main trial of Eni and Shell for corruption opened in Milan in the spring.
Both oil companies deny any wrongdoing.
Italian magistrates suspect the two oil groups used bribes to obtain rights in 2011 to OPL245, an offshore oil block estimated to hold nine billion barrels of crude, for $1.3 billion (1.1 billion euros).
Of that amount, $1.092 billion is believed to represent bribes paid to a London bank account that ended up going to various Nigerian politicians, including former oil minister Dan Etete.
The Nigerian government received only $210 million.
On Thursday, the Italian judge ordered $98.4 million seized from Obi, and 21 million Swiss francs ($21.8 million, 18.6 million euros) from Di Nardo after finding them guilty of "international corruption".
Their four-year prison sentences were slightly less than the two five-year terms sought by the prosecutor.
But Barnaby Pace, a campaigner at anti-corruption group Global Witness, nevertheless hailed the convictions.
"Today sees the first men fall in the murky Malabu scandal," he said in a statement issued Thursday.
"As Shell and Eni?s trial looms, time will tell whether it?s just the middlemen who pay the price for this epic crime against the Nigerian people," he added.
"But one thing?s for certain: this judgment will send shivers down the corporate spines of the oil industry -- and will surely alarm Shell and Eni employees and shareholders who have been repeatedly told that there was nothing amiss with the OPL 245 deal."
According to Radiocor, the Italian economics information agency, Obi unwittingly revealed the affair by suing Etete in a London court for payment of his commission for setting up the deal.
He won that case in 2013, receiving $114 million, of which $24 million was paid to Di Nardo.
The main trial brought by Nigeria against Eni and Shell has scheduled its next hearing on September 26.
A total of 13 people face charges, including Etete, Eni boss Claudio Descalzi and his predecessor, Paolo Scaroni.
Eyewitness? Submit your stories now via social or:
Email: eyewitness@pulse.ng
