אֵ֣ת כְּדָרְלָעֹ֜מֶר מֶ֣לֶךְ עֵילָ֗ם וְתִדְעָל֙ מֶ֣לֶךְ גּוֹיִ֔ם וְאַמְרָפֶל֙ מֶ֣לֶךְ שִׁנְעָ֔ר וְאַרְי֖וֹךְ מֶ֣לֶךְ אֶלָּסָ֑ר אַרְבָּעָ֥ה מְלָכִ֖ים אֶת־הַחֲמִשָּֽׁה׃
against Chedorlaomer king of Elam, and Tidal king of Goiim, and Amraphel king of Shinar, and Arioch king of Ellasar; four kings against the five.
Rashi on Genesis
'ארבעה מלכים וגו FOUR KINGS [AGAINST THE FIVE]— and yet the fewer were victorious; this statement is expressly made to tell you how powerful they were, and yet Abraham did not refrain from pursuing them.
Siftei Chakhamim
Yet, the fewer ones were victorious. Rashi is answering the question: Why does it need to state, “Four kings”? We see that they were four! Thus he explains [that it comes to teach]: “Yet ... Avraham did not refrain...”
Chizkuni
כדרלעמר, he is mentioned first at this point as he was the cause of the entire war.