The BL 4 is a very nice gun. These were imported by Garcia. The BL series are a fantastic field gun. They are good for informal clays ,a bit light for serious comp. shooting.
I'm sad to say if your dad paid $800 for this gun new in the early 70's he got ripped big time. They could be had for $350-375 most places with $425 being the high end.
Today the same gun in 95% condition are sold in the $550-650 range with a max of $700.