The Liberator IV was apparently the designation given to the B-24E for RAF service, but no squadrons appear to have used that aircraft. Other sources suggest that a small number of B-24Gs and B-24Hs might have been given this designation.
Consolidated B-24 Liberator (Crowood Aviation), Martin W. Bowman. A well balanced book that begins with a look at the development history of the B-24, before spending nine out of its ten chapters looking at the combat career of the aircraft in the USAAF, the US Navy and the RAF.