Acts as a chaperone for the L4 ribosomal subunit encoded by rpl4A and rpl4B, 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. Distributed throughout the cell but is enriched in the nucleus. Belongs to the ACL4 family.