501(c)(4) refers to a section of the U.S. federal income tax code. Corporations that have been granted 501(c)(4) status by the Internal Revenue Service may engage in political lobbying. This includes donations to political committees that support or oppose ballot measures, bond issues, recalls or referenda.
Donations to 501(c)(4) organizations are not tax-deductible.
501(c)(4) organizations are not required to publicly disclose their donors. They may choose to do so, or the donor may choose to disclose that he or she provided a grant to a 501(c)(3) organization.
Some non-profit groups have two related corporations -- a 501(c)(3) and a 501(c)(4). Under IRS tax code, it is legal to transfer funds from a 501(c)(3) to a 501(c)(4), but the restrictions on how the money is spend carry over on any such transferred funds.