What are attorney fees recoverable under section 1021.5 of the California Public Interest Act?

"Attorney fees are recoverable under section 1021.5 (1) by a successful party, (2) in an action that has resulted in the enforcement of an important right affecting the public interest, (3) if a significant benefit has been conferred on the general public or a large class of persons, and (4) the necessity and financial burden of private enforcement are such as to make the award appropriate. The statute's purpose is to encourage public interest litigation that might otherwise be too costly to pursue. [Citation.]" (Bowman v. City of Berkeley (2005) 131 Cal.App.4th 173, 176.)