All four of them are still sniffling and sneezing, despite a 10-day course of Clavamox. I am almost certain the whole thing is viral, but at least the Clavamox has ensured no secondary infections.
On a less positive note, it seems our elderly cat, Cosmo (age 15) has caught the URI. He started out "reverse sneezing" so we took him off to the vet. He *hates* being in the car, and the vet is nearly 30 minutes away. He was NOT a happy boy, but the vet told us his lungs were clear and he seemed healthy. However, the next evening he was extremely lethargic and when I did a skin pinch test, he was clearly dehydrated. So, off to the emergency vet service. The vet there said he had a fever. He had to stay overnight to get rehydrated, and to get a urine sample for culture.
We brought him home yesterday and quarantined him. So now two rooms in our house are "cat quarantine" rooms
All in all, despite Cosmo's illness, I think the situation balanced out positively. Now I just need to nurse my boy back to health.