Involved specifically in the folding of NDHH, a subunit of the chloroplast NADH dehydrogenase-like complex (NDH). Part of the Cpn60 complex composed of 7 alpha and 7 beta subunits. Can also form a complex composed of 14 beta subunits only. Both complexes show ATPase activity. The Cpn60 complex interacts with the Cpn10 complex. Interacts with NDHH. Up-regulated by light. The C-terminus (568-611) is required for efficient NDHH folding, but not for the formation of the chaperonin complex. No visible phenotype besides impaired NDH activity. CPN60B1, CPN60B2 or CPN60B3 cannot complement the function of CPN60B4. Assisted protein folding requires ATP hydrolysis, but not K(+) ions. Belongs to the chaperonin (HSP60) family.