there shouldn't be any plastic items in a chinchilla cage, as they can chew and ingest the plastic and end up with impaction/bloat/potential stasis. i'd remove the dust houses and plastic flying saucer asap.
as Megg mentioned, cover with fleece or replace all open wire shelves and ramps with kiln dried pine. chinchillas don't really need ramps anyway, they jump from level to level and are quite good at it.
i don't see any hay in your cage? that is an essential food item we need to supply to chins, to ensure they get the high fiber they need in their diet and to keep the teeth worn down.
how did you connect the two cages? i have a similar style of cage and would like to know how i might connect two of them together some day too!
as for introducing a fourth chin to your already bonded trio, i wouldn't advise it. adding that fourth chin can break the bond the three original chins have, and could potentially mean all of them will need to be caged separately due to fighting. the great thing about your setup is if one of your three chins gets sick or needs to be separated for any reason, you already have two cages that can be separated into different levels/compartments easily! lol.
last thing - you should have more than one water bottle and one food dish. multiple chins in one cage and only one place to eat/drink can mean the less dominant chin(s) not getting enough to eat or drink due to bullying.
Im now looking to add another female since i have the room.
