Goldman analyst Richard Ramsden said the four money center banks – Bank of America , JPMorgan Chase , Citigroup  and Wells Fargo – lost $28 billion in market capitalization last month during earnings season, when investors fled the banks stocks fearing huge losses on bad mortgage paper.

Yet the four biggest U.S. banks collectively face just around $26 billion worth of losses on so-called putbacks of private-label mortgage securities, by Ramsden’s count.

A putback happens when an investor in the mortgage-backed bonds sold by Wall Street concludes the selling bank hasn’t lived up to the promises it made in the offering documents, and presses the bank to repurchase delinquent mortgages that ran afoul of underwriting guidelines, for instance. Private-label mortgage securities are those sold by banks without the involvement of the government-sponsored mortgage companies Fannie Mae and Freddie Mac.

The four biggest banks sold $1.3 trillion worth of loans underlying such securities, some 40% of which have already been paid off, according to Goldman’s reckoning. That leaves some $810 billion worth of loans outstanding, more than half of which are with Bank of America.

That is a lot of loans, but Goldman expects the losses to be minimal, even in what it deems a worst-case scenario. It forecasts $15 billion of losses at BofA, $5 billion at JPMorgan Chase, $3 billion at Wells Fargo and $2 billion at Citi.

That’s based on an assumption that 28% of so-called conforming loans will fall into serious delinquency, meaning more than three months of missed payments, and that investors will put back 60% of those delinquent loans to the banks. Half those putback attempts will succeed and the banks will book 60% losses on loans they are forced to take back, according to Goldman’s model.

But the process will take time, to say the least, which will play in the banks’ favor and make the mortgage putback story a hit to earnings rather than a stress on capital, Ramsden says.

“We estimate the potential for $26 bn of losses that would be spread out over several years, relative to the $28 bn the market has already implied,” he writes.

Ramsden remains bullish on the banks – he rates BofA buy and has both JPMorgan and Citi on his conviction list with buy ratings, while he rates Wells Fargo neutral – but he did trim his price targets on BofA and Wells, citing weakening 2012 earnings prospects.