One of the primary rRNA binding proteins, this protein initially binds near the 5'-end of the 23S rRNA (PubMed:3298242). It is important during the early stages of 50S assembly (PubMed:3298242). It makes multiple contacts with different domains of the 23S rRNA in the assembled 50S subunit and ribosome (PubMed:7556101, PubMed:6170935). Protein L4 is a both a transcriptional repressor and a translational repressor protein; these two functions are independent of each other. It regulates transcription of the S10 operon (to which L4 belongs) by causing premature termination of transcription within the S10 leader; termination absolutely requires the NusA protein. L4 controls the translation of the S10 operon by binding to its mRNA. The regions of L4 that control regulation (residues 131-210) are different from those required for ribosome assembly (residues 89-103). Forms part of the polypeptide exit tunnel. Can regulate expression from Citrobacter freundii, Haemophilus influenzae, Morganella morganii, Salmonella typhimurium, Serratia marcescens, Vibrio cholerae and Yersinia enterocolitica (but not Pseudomonas aeruginosa) S10 leaders in vitro. Part of the 50S ribosomal subunit (Ref.1, PubMed:7556101, PubMed:3298242, PubMed:10094780, PubMed:12809609, PubMed:11511371, PubMed:16272117, PubMed:25310980, PubMed:24844575, PubMed:27934701, PubMed:27906160, PubMed:27906161). The erythromycin sensitive allele is dominant over the resistant allele. Belongs to the universal ribosomal protein uL4 family.