Theres definitely plenty of space to add more fish if you have 4 venustus in ~120 gallons of water. You just have to be careful about your selection. Venustus get big, and can be somewhat aggressive, but aren't murderers for their size. You want to add fish that will get almost as big or bigger, but aren't too aggressive. Blue dolphins (Cyrtocara Moorii) or VC-10 (Placidochromis milomo) would be good options and are somewhat readily available. Most mbuna, other than bumblebee, that get bigger than 6" will likely be trouble if you add them to the tank. The smaller ones would eventually be eaten most likely.