(interrogative) what: e.g. (1) asking for a subject complement; (2) asking for a sentence object
(relative) what: e.g. (1) as the object of a sentence; (2) ditto
(relative) that: e.g. (1) modifying an indefinite pronoun like iets, niets, alles or het enige; (2) modifying an adjective that is used as a noun, usually a superlative
(relative) which: e.g. (1) modifying the demonstrative pronouns dat and datgene; (2) referring back to an entire sentence
(indefinite) something: e.g. (1) as subject; (2) as subject complement