Ribosomal protein S4 (RPS4) is one of the 21 proteins which belong to the 30S of the bacterial ribosome or the 30 proteins which belong to the 40S small subunit of the eukaryotic ribosome. It binds to the 16S RRNA[1]. Mutations in RPS4 are known to increase translational error frequency. In archaea the protein is named S4E or S4P. In eukaryotes it is named S9.