Beet soil-borne mosaic virus (BSBMV) is a member of the Benyvirus genus together with Beet necrotic yellow vein virus (BNYVV), with similar genomic organization and both vectored by Polymyxa betae. The ability of BNYVV helper strain to replicate BSBMV RNA-3 suggests a common and conserved viral RNA selection mechanism for both viruses. We recently described a 1,733 nts long BSBMV RNA-4 (GenBank: FJ424610), which has been molecularly and functionally characterized. As for BSBMV RNA-3, fulllength BSBMV RNA-4 cDNA clone permitted the obtention of infectious transcripts that BNYVV viral machinery is able to replicate and to encapsidate in planta. Moreover, such BSBMV RNA-4 can substitute BNYVV RNA-4 for an efficient transmission through the vector P. betae in Beta vulgaris plants. Two putative ORFs have been identified and could encode for peptides of 32 kDa (383-1,231 nts) and 13 kDa (885-1,241 nts) respectively. Using BNYVV helper strain, BSBMV RNA4’s protein initiation codons have been studied by mutagenesis. We associated the local necrotic lesions phenotype to the protein expression onto mechanically inoculated Chenopodium quinoa plants. Flag or GFP-tagged sequences have been expressed in viral context. Western blot analyses of local lesions contents, using FLAG-specific antibody, revealed a high molecular weight protein, which suggest either a strong interaction of BSBMV RNA4’s protein with host protein(s) or post translational modifications that need to be better investigated. GFP-fusion sequences permitted the sub-cellular localization of BSBMV RNA4’s proteins.