All of the TES and FO3/4 games have been using the same engine since Oblivion. Bethesda now calls their internal build of GameBryo "Creation Engine" but it's very much the same thing. I think you can actually still pull "GameBryo" out of the EXE using the Unix `strings` util, and you can definitely see it from the game data.
So "ported to the FO4 engine" doesn't really mean much. It's like saying that Half-Life 2 has been ported to CS:GO's engine. Yeah, it's a more modern build of the engine but it's still the same engine and it's not going to look or run drastically different unless the assets are also revamped at the same time.
Also, my personal opinion (from the outside) is that Bethesda doesn't seem to have done a good job maintaining their codebase. Fallout 4 still doesn't really look any better than Skyrim (the facial animations are especially terrible), it's quirky/buggy as hell (framerate-locked physics in 2016?) and it runs like shit even on high-end machines. An engine from a game released in 2006 shouldn't struggle to hold 50 fps at 1440p on a 780 Ti. TF2 gets slagged on a lot for its shitty codebase but it's nowhere near that bad.