"We're talking about a balanced portfolio. We're talking about over a 10-year horizon," Fink said on CNBC's "Squawk Box."
"Obviously, this year we are making far better because of the equity market return," he added, but warned some years could be much worse. "There are going to be some times where you're going to have a market setback where you could lose 20, 30, to 40 percent of the market."
BlackRock is the world's biggest money manager with nearly $6 trillion in assets under management.
Fink was reiterating a prediction he made during a panel discussion at a Saudi investor summit moderated by CNBC's Andrew Ross Sorkin earlier Tuesday. His comments raised eyebrows because investors have come to expect bigger investments returns.
The S&P 500 has returned 7.6 percent annually during the last 10 years, a period that includes the financial crisis.
Fink said on CNBC that BlackRock researchers actually published the 4 percent return estimate two months ago. But since he talked about it at the conference it's getting attention.
The BlackRock chief appeared from Riyadh at the Future Investment Initiative, a summit presented by the Saudi Arabia's massive sovereign wealth fund to showcase investment opportunities as the kingdom aims to diversify its economy to be less oil dependent.
The transformation of Saudi Arabia's economy is "pretty impressive," Fink said.
"It's a whole new ballpark," Fink said. "Saudi is underinvested for over 20 years. They began the investment process over the last five years."
"They're really trying to push the whole society forward."
Fink also said the global economy is starting to see acceleration. "We're in our eighth year from the Great Recession," he said. "We're seeing Japan and China accelerating. Japan may grow 2½ percent over the next quarter."
If President Donald Trump passes his tax reform plan in the first quarter of next year, Fink added, the U.S. market will see "another leg up."