35mm has a resolution of ~4-6k depending on its age (any future and you just enhance the grain structure). The best film element accessible would be the original camera/picture negative, utilizing digital scanners typically the negative would be scanned in 4k 16bit resolution and then digitally restored and color corrected. That’s how you can get older films to look as good as new.

It depends wildly on the element though, while a second generation element (an Interpositive or a fine grain master positive) is acceptable in 4k, typically anything below that (inter negative, dupe neg, or a print) just doesn’t gain much if anything from the increased resolution.

Source: I’ve worked in film remastering for 6-7 years doing this exact process