plcmt 4 means No Content/Standalone
A video ad with no content video, placed in a feed, slideshow, article body, or floating unit.
When to send it
Send 4 for a video ad that runs without any streaming video content and is not an interstitial: an outstream unit in an article, a video ad in a native or social feed, a slideshow slot, or a sticky or floating player that carries only ads.
Buyers treat 4 as outstream and price it on viewability and completion rather than on content data. The IAB implementation guide's example pairs old Video.placement 3 (In-Article) with Video.plcmt 4. It is confused with 2 when an article player also shows a content clip, which makes it accompanying content, and with 3 when the unit covers the screen and cannot be scrolled away. Because there is no content, Video.startdelay has nothing to describe here.
Example
{
"imp": [
{
"video": {
"plcmt": 4
}
}
]
}Fields that carry it
Other values in plcmt subtypes (video)
The plcmt subtypes (video) list page explains the whole list and its common mistakes.