This paper reports on a case study of a quadrilingual child, Stefan, born and raised in Sweden and exposed to four languages before his first birthday: English, French, Russian and Swedish. We examine his vocabularies in these languages by the Cross-Linguistic Lexical Tasks (Haman, Łuniewska, & Pomiechowska, 2015), designed to measure vocabulary in monolingual and multilingual children. Stefan’s scores on comprehension and production reveal proficiency in all his languages, to varying degrees, and with comprehension exceeding production. While highlighting direct and indirect exposure as explanation for the variation in proficiency, we also discuss cognate vocabulary as an important factor for multilingual language development. In the production tasks, Stefan demonstrates not only vocabulary knowledge but also language-specific use of morphosyntax.