an algebraic expression containing four terms.

Is a Quadrinomial a thing?

an expression of four terms.

Is Serendipity a real word?

Serendipity is a noun, coined in the middle of the 18th century by author Horace Walpole (he took it from the Persian fairy tale The Three Princes of Serendip). The adjective form is serendipitous, and the adverb is serendipitously. A serendipitist is "one who finds valuable or agreeable things not sought for."