Bartók's fourth Quartet exemplifies contradictory influences on a composer who, unable to abandon all reference to tonality, wants to avoid excessive folk connotations. According to Milton Babitt, Bartók's solution consists in a mild functionalism, veiled by a constant transformation of the thematic elements. The tonal construction of the Quartet appears to correspond to Erno Lendvaļ's description of the musical system of Bartók, both in the tonal relations and in their proportions. The paper reminds also comments by George Perle, Leo Treitler and Elliott Antokoletz on Bartók's language that find applications in this work. An analytical description is then proposed, envisaging the thematic organization and the "two harmonic worlds", chromaticism and diatoncism.