One of the primary rRNA binding proteins, it binds directly to 16S rRNA where it nucleates assembly of the body of the 30S subunit. With S5 and S12 plays an important role in translational accuracy. Part of the 30S ribosomal subunit. Contacts protein S5. The interaction surface between S4 and S5 is involved in control of translational fidelity (By similarity). Belongs to the universal ribosomal protein uS4 family.