As explained in An Introduction to Balancers, X, second and third chromosomes must have both multiple inversions and recessive lethal or sterile mutations to work effectively as balancers. Since meiotic crossovers normally do not occur between fourth chromosomes, inversions are not necessary for a fourth chromosome to act as a balancer. Any fourth chromosome with a recessive lethal or sterile mutation can serve as a balancer for another fourth chromosome.
The recessive lethal, fourth chromosome mutations that work best for balancing other mutations are those that are also associated with easily scored phenotypes. Here we have listed a few convenient classes of recessive lethal mutations.