Confronted to the unusual musical upheavals of the beginning 20th century, Sibelius follows his own way and develops a highly personal style, of which modality is one important characteristic. Yet, most of Sibelius's works are not "modal" strictly speaking: often, they conform to a language and forms stemming from common practice tonality. The Finnish composer often combines tonality and modality, but not always in the same way. The links between the two types of language appear multiple and complex. Modality may either oppose tonality, or on the contrary enrich it. On the basis of a comparison of the Fourth Symphony with Luonnotar, the present paper aims at defining "functional levels" of modality and at evidencing common points in the pitch systems of the two scores.