Private equity has been largely the province of ultra-rich investors, endowments and pension funds. That may be changing.

Balances for 401(k) accounts averaged $131,700 in the fourth quarter, according to Fidelity. ... FILE - Fidelity Investments logo is seen on a building in Manhattan, New York.

Wall Street firms have been pushing to get private investments into the hands of individual investors, and they see the market for 401(k)-type retirement plans crucial to this growth.