What happens in New Vegas stays in New Vegas, they say, though New Vegas itself will not stay in Fallout: New Vegas. A new mod named Project Mojave is attempting to recreate a lot of the post-apocalyptic Sin City and surrounding areas as a Fallout 4 mod, and you can play the first slice now. But to manage your expectations: it is not an attempt to recreate Obsidian Entertainment's game Fallout: New Vegas inside Fallout 4.
The modders explain that they "intend to open and recreate some of Fallout New Vegas's map as a new explorable location as a addition to Fallout 4", a hub world sandbox sorta thing. They say they "aim to faithfully recreate and remake the Mojave Wasteland true to the original and open to the public to play with, explore, mod and build." So when it's done, it could be the foundation for interesting newness.
You can download the initial "early access" version from Nexus Mods. Expect the New Vegas strip with playable casino games, about a quarter of the map, a few spots to build your own home, and creatures and weapons and things.
This is very much New Vegas the city, not Fallout: New Vegas the game. The mod is even explicitly set after F:NV and during the events of F4. This is not another like Skyblivion or those other mods remaking old Elder Scrolls games inside newer ones. That said, another team are attempting to do that with New Vegas.
The Fallout 4: New Vegas mod team have been working for several years to recreate the whole game, and after a few months of silence have started showing progress on their Facebook page again. I don't know if any of these sorts of remake mods will ever be finished (or evade the wrath of Bethesda's lawyers) but they're working on it and it looks shiny. Here's a F4:NV video from 2020:Ta to PCGamesN for pointing out Project Mojave.