The 4' lengthy is pretty limiting imo. I don't think a jag is appropriate for tank like that.
Something that stays around 12" is think is better suited. I understand it's a large volume of water but the footprint is not the best.
What about a Cuban cichlid? Or a rivulatus?
Try rapps, coa or don conkal... be average temperament some may be aggressive but will tolerate a birchir unless your very unlucky. Growth will be about average. Im not an expert but have done a little research, they are very rare in the uk, would like to try one one day.