New Vegas and fo3 used the same engine, a couple minor changes but the overall structure was the same, so porting the stuff over was very easy, but the changes to engine and gameplay broke somethings, speech checks, Damage Threshold instead of Damage Resistance. Tale of two wastelands isn't just porting things over, it's rebuilding the stuff that didn't fit the system changes. for Skyrim, Bethesda built a new engine, there are similarities to the old one but the backend was probably massively redesigned. Fallout 4 uses an updated version of Skyrims engine, which again is very different to New Vegas' engine. Ignoring the Art design differences, and how jarring they'd look, (coming across one of those crap looking powder gangers outfits alongside the stuff in fo4, not a pretty sight), the systems are incredibly different, Dialogue has massively changed, armour works differently, back to Damage Resistance etc. Simply put, it might be possible, but it would need a huge team, a huge amount of work, and there's a chance most of the stuff would have the be rebuilt, just take a look at how long tale of two wastelands has been in development, and they have a decent sized team for a mod, this mod would have to be more of a complete rebuild like Skywind, and you can see how long that's taking.