Co-chaperone that binds to the molecular chaperones Hsp90 and Hsp70. Stimulates Hsp70 ATPase activity, but not Hsp90 ATPase activity. Involved in only a subset of Hsp90 functions (By similarity). Monomer. Component of Hsp70 and Hsp90 chaperone complexes (By similarity). Belongs to the TTC4 family.