Which Scandinavian language has preserved the four-case system of Old Scandinavian?

The Scandinavian language that has preserved the four-case system of Old Scandinavian is Icelandic.12

While other Scandinavian languages have undergone significant simplification of their grammatical structures over time, Icelandic remains the most conservative of the group. It continues to utilize the four original cases—nominative, genitive, dative, and accusative—alongside a complex system of genders and declensions that has changed very little since the classical period. In contrast, Faroese has simplified its system to three cases in speech, and the remaining Scandinavian languages have largely lost these case distinctions, retaining them only in certain personal pronouns.12