It really depends on what kind of film stock it was shot on. That information isn't publicly available though, only those who were involved with the shoot would know.
I do believe Ghosts was shot on a type of 35mm film, it just depends if it's normal or super 35mm (the latter of which came about in 1982. It was adopted by Hollywood in 1984, so too late for Thriller). Super 35mm allows more image on the side as the space on the film strip often reserved for optical audio is instead used for the visual itself. Generally this means you can restore to a fuller picture in 16:9 without cropping the top and bottom. On normal 35mm, you're generally going to lose some of the top and bottom although possibly gain some on the side. It really varies production by production and how they've chosen to go about it.
Watching the Ghosts music video on YouTube, there are a number of shots that would have essential information missing if it was cropped from 4:3 to 16:9. I don't know if they shot intending it to be display in 4:3 (very possible, given the fact it would mostly be played on MTV in 4:3), or if they shot it, protecting for widescreen, and just cropped it to 4:3.
Either way I don't mind what aspect ratio they release it in, so long as it respects the Director of Photography's intent. Nothing wrong with 4:3, in fact many great films are in 4:3 (although they're generally a lot older).
If they have access to the film negatives for Ghosts, then yes they can make a 4K version. Thriller 3D was worked on at 4K I believe.
