The International Automobile Federation (FIA) said in a statement on Wednesday that power units homologated (approved for use) in previous seasons could now be re-homologated for 2016.
"Previously no manufacturer could supply more than one specification of power unit," the FIA said.
"The World Motor Sport Council was also advised that the FIA had agreed for Ferrari to supply a fourth customer team with a 2015-specification Power Unit in 2016."
A formal deal has yet to be announced.
Ferrari are already scheduled to supply Sauber and the new U.S.-based Haas team as well as their own works entry.
Toro Rosso, who used Ferrari engines from 2007-13 and took current Ferrari driver Sebastian Vettel to his first Formula One victory at Monza in 2008, need a change after owners Red Bull fell out with under-performing Renault.
Red Bull are expected to continue using an engine provided by Renault for one more season after other manufacturers rejected approaches.
The FIA also said a similar level of in-season engine development would be allowed in 2016 through the current system of 'tokens', which had been due to go down to 25 but would remain at 32.
The allowance will then drop to 25 in 2017 and 20 in 2018.
The decision will help McLaren's partners Honda who returned to the sport this year but struggled with poor performance and a lack of reliability.
The World Motor Sport Council also approved, with just one vote against, a mandate for the FIA president Jean Todt and the commercial rights holder Bernie Ecclestone to make recommendations and decisions "regarding a number of pressing issues".
They included governance of the sport, power units and reducing costs.
"Mr Todt and Mr Ecclestone expressed their intention to establish conclusions on these matters by 31 January, 2016," the statement said.
(Editing by Ed Osmond)
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