Finches don t play with toys as much as parakeets and parrots, but still are curious and like to put their beak into things. A sturdy string of beads hanging the from the top, a plastic ball on the floor, etc. Try different things. They may ignore, may enjoy it. It is recommended that only one pair be kept in the same cage.
Zebras have a hierarchical society, as do many animal groups.
When you put two pair together they may fight for the top position and feather pick each other. This situation does not promote breeding and of course is not healthy. If you want more birds you will need multiple cages or a larger cage. With a large enough cage you can certainly have multiple pairs together. This is the minimum for three pair.