Quatre-vingts, quatre-vingt-un, quatre-vingt-deux... quatre-vingt mille, quatre-vingts millions. This doesn't affect pronounciation in general, because this s is silent... except in the context of liaisons, such that you would say "quatre-vingts (z')avions" and not "quatre-vingt (t')avions", although in colloquial speech, nobody cares. In written form, I expect many of my fellow French native speakers to do the mistake or at least be uncertain. We mostly write it "80" anyway.

Spelling depends on various things. If it's followed by another number, then its singular: quatre-vingt-trois. Otherwise, it's plural, quatre-vingts. Unless it's an ordinal / noun, J'habite au quatre-vingt rue de la Paix.

Pronunciation is the same, unless there is a liaison: elle a quatre-vingts ans = there will be a 'z' sound between vingts and ans.

FYI, I'm a native speaker and I had to look the spelling part up to be sure. There are other rules with other exceptions for cent, mille, million, milliard.