Infact for Cichlids, tank floor space is more important the gallons. I'd guess the capacity of a 4 foot to be between 40 and 60 gallons, depending on the other dimensions.
Kribs might work in the tank, but you'll have to be aware that they are as territorial as your apisto's when breeding, Make sure if you do mix them that you have plenty of spare caves and spawning sites for the kribs, they are similar to apisto's in care terms, but are larger so they might bully the cockatoos if there is a fight over territory. I would also add tall plants to break up the space, and give hiding spaces for the fish if they are being chased.
In general for Dwarf Cichlids i would also add a small shoal of dither fish to your tank. Not small tetras as they may be eaten by the angel, but larger tetras like cardinal or emporer tetras would work well. Pencil fish are popular with dwarf cichlids as they are slower swimmers and less likely to eat any fry you may have from the kribs or cockatoos.