A probably minor shell protein component of the carboxysome, a polyhedral inclusion where RuBisCO (ribulose bisphosphate carboxylase, rbcL-rbcS) is sequestered. The central pore probably regulates metabolite flux, as might the gaps between assembled homohexamers (PubMed:16081736). Homohexamers make sheets that probably form the facets of the polyhedral carboxysome (Probable) (PubMed:19844993). This subunit probably makes both homohexamers and heterohexamers with CcmK3 (Probable). Homohexamer (Probable) (PubMed:19844993, PubMed:31603944). Interacts with full-length CcmM (PubMed:17993516). Forms mixed heterohexamers with CcmK3, probably with 1:5 CcmK3:CcmK4 stoichiometry. Only very weak interactions with CcmK1 and CcmK2 were seen (PubMed:31603944). This cyanobacterium makes beta-type carboxysomes. The tight homohexamer forms a pore with an opening of about 4 Angstroms in diameter which is positively charged. Belongs to the bacterial microcompartments protein family. CcmK subfamily.