My 6 month old was unknowingly exposed to chicken pox 4 days ago. what are the chances with no symptoms that he gets them?

Possible: If there was direct contact with the blisters, lesions, there is a high chance. hard to give a percentage. Call your peds to check there is a fever and a red rash for starters. Babies tend to get milder illness, but there is no such guarantee. Also too young for the vaccine, as you might know.

Hard to predict: Half an hour in the same room with a case is enough to transfer the germ by air. The incubation period is 10-21 days before the disease will appear if he gets it. The chances here are unknown but real.If you had chickenpox as a kid, there may be a tiny bit of protective maternal antibody (from pregnancy) but most of it is gone.Let his doc know if he gets it.Most are mild at that age.