One of two assembly initiator proteins for the 30S subunit, 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; many suppressors of streptomycin-dependent mutants of protein S12 are found in this protein, some but not all of which decrease translational accuracy (ram, ribosomal ambiguity mutations). Plays a role in mRNA unwinding by the ribosome, possibly by forming part of a processivity clamp. Protein S4 is also a translational repressor protein, it controls the translation of the alpha-operon (which codes for S13, S11, S4, RNA polymerase alpha subunit, and L17) by binding to its mRNA. Also functions as a rho-dependent antiterminator of rRNA transcription, increasing the synthesis of rRNA under conditions of excess protein, allowing a more rapid return to homeostasis. Binds directly to RNA polymerase. Part of the processive rRNA transcription and antitermination complex (rrnTAC). The complex forms an RNA-chaperone ring around the RNA exit tunnel of RNA polymerase (RNAP). It supports rapid transcription and antitermination of rRNA operons, cotranscriptional rRNA folding, and annealing of distal rRNA regions to allow correct ribosome biogenesis. This subunit may play a particular role in long-distance rRNA annealing needed for pre-rRNA processing. Part of the 30S ribosomal subunit (PubMed:4587210, PubMed:1100394, PubMed:7556101, PubMed:12809609, PubMed:16272117, PubMed:27934701, PubMed:10094780, PubMed:12244297, PubMed:27906160, PubMed:27906161, PubMed:28077875). Contacts protein S5. With proteins S3 and S5 encircles the mRNA as it enters the ribosome, which may play a role in mRNA helicase processivity (PubMed:15652481). Some nascent polypeptide chains are able to cross-link to this protein in situ (PubMed:9716382). The rRNA transcription and antitermination complex (rrnTAC) consists of RNAP, NusA, NusB, NusE (rpsJ), NusG, SubB, ribosomal protein S4, DNA and precursor rRNA; S4 is more flexible than other subunits (PubMed:32871103). Belongs to the universal ribosomal protein uS4 family.