technically, it's just a matter of view port.
meaning, it shouldn't be a problem to have the the _actual_ fallout 4 rendered isometric.
fo3 HAD isometric and top down views you could switch to at runtime "onboard" (which btw pretty clearly proved why that's NOT that good an idea for a game that's pretty ranged by nature, and i'm saying this as a spoken iso-lover).
and you also still have their iso modes in all the editors up to skyrim's creation kit, and iso is natively supported by the creation engine / gamebryo (they made games like "sim city societies" on gamebryo).
so, if you can't still switch the _actual_ fo4 to top down / iso (i think it'll be possible anyhow though, it's just a nice feature to add at no cost, so why should they not), i'm pretty positive this should be moddable.