A 501c4 is a social welfare org. Political activities are generally prohibited

"Political or social activities. The promotion of social welfare does not include direct or indirect participation or intervention in political campaigns on behalf of or in opposition to any candidate for public office."

https://www.law.cornell.edu/cfr/text/26/1.501(c)(4)-1

I think this is a fair confusion currently however given their strong dem tie-in recently and their current approach of taking on individual politicians (for and against) by name.

Note: If you are active in the ACLU they have a related ACLU Foundation they use to collect 501c3 money, is run by the same people out of the same offices, and actually brings in more money I think than the c4 if that matters.