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The choices here involve frequency, just as you said in your post.
"Seldom" and "rarely" mean more or less the same. The former is more formal than the latter, and is usually used in written English.
To express frequency, "hardly" and "scarcely" would need "ever": "hardly/scarcely ever" means "almost never".
Finally, the right choice IS 4, with one more detail to know. That is, words of frequency are not used to end a sentence. With the verb "be", we say "Los Angels rarely is" (but "Los Angels is rarely cool" when words of frequency are not the last word of a sentence).