You deleted your message text so you probably got your answer on your own, but for the benefit of others...
(Edited to correct misinformation)
Paramount is owned by ViacomCBS.
Yellowstone was released on Viacom's Paramount Network cable/satellite channel. For streaming, they decided to license Yellowstone to NBC Peacock rather than their own Paramount streaming app in the hopes of reaching a broader audience and to obtain the licensing fees. This was before the Paramount+ streaming service was introduced.
Peacock currently has the first 3 already-shown seasons, but season 4, currently being released one week at a time, is only available on its original source, the Paramount Network on cable/satellite.
I expect the whole of season 4 will be available on Peacock when it has been completely shown on Paramount. The whole thing may come home to Paramount+ when the licensing deal with Peacock expires.
Peacock has announced it will air season 4 of Yellowstone beginning March 28, 2022.
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