By Svea Herbst-Bayliss and Jennifer Hiller

BOSTON (Reuters) -The California Public Employees' Retirement System (CalPERS) said on Monday it plans to vote for Engine No. 1's four director nominees to Exxon Mobil Corp's board, throwing additional support to the activist hedge fund in its proxy fight with the company.

"CalPERS is supporting additional board refreshment due to the long-term financial underperformance at ExxonMobil and the need for a greater depth of skill sets and experience on the board to address the significant challenges the company faces," the pension fund said on Monday.

The fund is weighing in on what promises to be this year's most closely watched board room challenge where Engine No. 1, a newly launched hedge fund, is taking on one of corporate America's most iconic companies and pushing for improved financial performance and a greater focus on clean energy.

CalPERS said it would back all of Engine No. 1's four nominees because it feels the board would "benefit from additional expertise in both its core business and in renewable energy technologies."

Engine No. 1's slate includes Gregory Goff and Anders Runevad, former chief executives of oil refiner Andeavor and wind-turbine manufacturer Vestas Wind Systems, respectively; Kaisa Hietala, former head of renewable fuels at Finnish refiner Neste; and Alexander Karsner, the former U.S. assistant secretary of energy for efficiency and renewable energy.

(Reporting by Svea Herbst-Bayliss and Jennifer Hiller; Editing by Chris Reese and Peter Cooney)