It could be a telescoping downdraft, or perhaps they opted not to have one if they don't do high heat cooking.

See that strip of stainless behind the cooktop?
That's the retractable downdraft vent.

We are installing a Viking 4 burner rangetop with a Vent-a-hood ventilation hood over the cooking island. We were afraid the downdraft just wouldn't provide adequate ventilation. We currently have a totally ineffective Jenn air downdraft and are gun shy. Any opinions/thoughts?

Did you end up doing a downdraft and have you been happy?

The downdraft systems will never be as effective as a regular vent hood, especially if you do serious cooking. There is a new downdraft that is slightly more effective because it comes 18" up from the counter top. You would do any higher moisture cooking on the back burners to increase ventilation effectiveness.