"Challenge" may have been a poor choice of wording on my part.
ECL is "effective character level", and the same number as level in typical cases. How well ECL actually handles the atypical cases is a point of much contention; but that's the number used in situations like these.
A single creature of a given CR is indeed supposed to be challenging to a party of four characters of the equivalent ECL; but that comes with the definition of "challenging" meaning the party is expected to win with competent play, but consume about one-fifth of their resources (hit points, spells, etc.) doing so.
By "a challenge" I meant that (without knowledge of starting conditions) both sides could reasonably expect to win, there's no foregone conclusion....Which is also about where four creatures of a given CR would rate against a party of four characters of the equivalent ECL.