What is "fourth-party" in advertisement?

Question

While I understand third-party cookies, I am not sure to understand what exactly "fourth-party" means in sentences such as:

Third and fourth party ad tracking is allowed on [website] subject to the following guidelines and policies.

Media Company does not have visibility into Agency’s deployment of Fourth-Party Ad Tags

[...] privacy and compliance risks that can come from the use of third and fourth-party ad vendors.

Question: In the web advertisement world, what does "fourth-party" usually refer to?

Answer 1

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 YouTube video.

Answer 2

Google's documentation cites these examples for what they generally consider as fourth-party:

The vendors permitted to make fourth-party calls are generally of the following type:

• Research products, which include Analytics/Performance, Brand-Lift Studies, & Verification Services.

By opposition to third-party:

The vendors permitted to make third-party calls are generally of the following type:

• Demand Side Platform, Agency Trading Desk, Ad Network, Ad Exchange, Standard Ad Server & Rich Media Vendors.