Great question — answering as the UphillTech team:
Fourth party refers to any number of script/pixel chains beyond your 3rd party pixel/scripts. In practice, anytime you add a third party script, that script can then load a another party's script or pixel that can track you. This is a fourth party ad tracking. Here's an example from Google and Doubleclick From https://www.reflectiz.com/the-cybersecurity-effects-of-fourth-parties-on-websites/ ... as part of your marketing efforts, you use an embedded YouTube clip to present your product on your website. But did you know that Google installs Double-Click pixels on your website? YouTube belongs to Google, and Google uses its advertising platform, Double-Click, to monitor who is watching your...
Full answer by Trebor: https://webmasters.stackexchange.com/a/131395 (CC BY-SA 4.0).
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