The footage is analog (on film). It was shot with 0 pixels, so 4k pixels on an edge doesn't matter. Side note, footage itself is a term derived from film (how many feet of film).

You can scan film into whatever digital resolution you want. You could do an 8k scan if you felt like it. You might run into issues where the resolving power of the film is less than the scan, but 4k is not an unreasonable resolution to pull out of well lit studio shot movie stock.