First- have you quarantined your new chins?
Second- Putting the new baby in the cage isn't a good way to go about it. Chins are territorial, so they will eventually need to be all put in either a cage none have been in before or a scrubbed down and rearranged cage to make them think this is a new shared territory and not someone's.
Third- are you making sure the chins are all the same sex? I just ask to make sure your thinking about it. You should never house opposite sex chins together unless one sex has been spayed/neutered or you are actively planning on breeding. Breeding can happen in an instant and is not something to take lightly.
Fourth-even if you do manage to make all four happy in the same cage, make sure your have separate cages for EACH chin in case they one day decide they don't like each other. It's been known to happen in happily bonded chins.
Fifth- they may never like each other. bonding chins is about fitting personalities together. Some people don't get along with other people and some chins will never get along.
Ok. Sorry, that came across mean. I dont mean to but there's a lot of things to keep in mind to make sure every chin stays healthy and safe. There are a few different methods to bond- some people prefer to have playtime together, other prefer to smoosh them all in a cat carrier (im not sure how that'd go with four). I would try introducing one of the new chins first and making sure it's accepted rather than trying both at the same time.
There's plenty of tips and discussion of bonding on the forum- use the search tool and browse the threads.
Welcome to the forum, and as I've said, read around. Browsing and searching threads will get you so much info.