The four banks, identified as the National Bank of Greece (Stuttgart: 876113 - news) (NBG), Alpha, Eurobank and Piraeus, will receive support from the state because they are considered crucial to the national banking sector, AFP reported.
A dozen smaller banks are to be recapitalised with private funds, also by the end of April, the national bank said.
"The Greek banking sector was severely hit over the past few years by the combined effects of the restructuring of Greek sovereign debt and adverse economic conditions, both of which affected banking assets and deposits," the central bank's report explained.
Its assessment of the bank's recapitalisation needs over the 2012-2014 period was "conservative" it said. A key factor behind the needs was a restructuring of Greece's sovereign debt, it said, which hit Greek banks hard because they held large amounts of sovereign bonds.
The banks were also expected to suffer losses on both domestic and international loans.
According to the central bank, the main objective of its "comprehensive banking sector strategy is to secure a well-capitalised and viable banking sector".
In addition, Greek officials seek "to gradually restore depositors and market confidence. The improvement in the capital and the liquidity position of Greek banks will enable them, in the medium term... to help improve the business environment and put the real economy back into a sustainable path," the report said.
Greece's main banks had been expected to receive €50bn overall to restore their capital after taking part in a sovereign debt writedown of more than €100bn in March, and a subsequent debt buy-back earlier this month.
European leaders have decided, meanwhile, to unblock €49.1bn from Greece's outstanding financial assistance package by early next year in return for a new austerity deal in Athens.
Of that amount, around €16bn is earmarked for aid to Greek banks, on top of €18bn already granted in May.
The ratings agency Standard and Poor's has given Greece's sovereign debt a six-point grade increase, while the European Central Bank has said it would again accept Greek bonds as collateral for central bank loans.