To what I've read, you should quarantine them from 4-8 weeks (I know, long time). Once you add any new fish into the quarantine tank the countdown start from the beginning, as new fish could infect fish in quarantine tank with whatever they have on them. If you planning on 2 more, I suggest you wait until this tank is empty, 4 discus, no matter how big in a 10G is too much, especially for such a long period of time. If you purchase all your fish in the same place/breeder that you can trust, it is pretty safe to say that if the fish carrying any diseases, all of his fish would share that disease. As most
LFS/breeders use 1 mega filter for all their tanks.
What you did is exactly what I did when i started with discus, because they cost a lot, you want to make sure everything is okay first. I started with 4 and 3 days later I bought another 3 from the same place, I just add them straight in without quarantine (only because they're from the same store), luckily the new fish didn't caught any disease within that 3 days. Now I've learned that the smart thing to do is buy all of them at once. They would
acclimate much better if allow to be in large number.