Like someone said earlier, you are good budgie mommy and i am sure you will be able to tell if they start to act differently or ill. Now you need to add a 5th male and you'll have a budgie ball team!
Can't you just see them with little budgie bats, mitts, caps, etc. LMAO
Sheri, I'm sure they'll all be ok. At least you've been able to observe Cayman for a little while already. Besides, if it was me and a new bird ended up sick, I would treat them all. I'm sure you'll make sure that they all do well and stay healthy.