Acts as a chaperone for the L4 ribosomal subunit encoded by RPL4A and PRPL4B, required for hierarchical ribosome assembly. Shields ribosomal protein L4 until timely release and insertion into the pre-ribosome is possible, once ribosomal protein L18 is present. Interacts with RPL4A and RPL4B. Distributed throughout the cell but is enriched in the nucleus. Present with 1630 molecules/cell in log phase SD medium. Belongs to the ACL4 family.