The 4 Cs stand for content, cookie duration, creative and context.
Content: This is the area of your site that somebody has visited. You don’t need to segment every page of your site here with its own audience and ad group. You want to create audiences that are grouped by the likelihood to convert.
Cookie Duration: For most sites, recent visitors have a higher likelihood of converting than less recent visitors. The right cookie durations to use for your remarketing audiences will depend on your product and funnel.
Creative: Refers to the type of ad that you are serving to your audience.
Context: Context refers to the placements on which your ad is appearing. Context is a strong signal of intent and readiness to move down the funnel; a person is much more likely to convert to a B2B software lead when they are browsing software-related content than they are when they are checking the weather.
CT for more details. Nice primer.