What rates of return are required to be reported to plan participants in a 401(k) plan with mutual funds and unitized company stock?

If the plan is registered under federal securities law, the prospectus is required to state trailing performance of each fund. I am not familiar with a registered plan that unitizes company stock.

If the plan is complying with the regulations under section 404© of ERISA, some document available from a plan fiduciary upon request will provide investment return information, probably the mutual fund prospectus and periodic fund statements and reports (if you have retail mutual funds). Derivative materials prepared by or for the plan may also state returns.

If the plan is not required to be registered and is not participant directed, the plan is not required to report returns on particular investments, but the plan investment fiduciaries will have investment return information.