Join the short Viator city tour. They have 9 am, 1pm, and 3 pm. The 1 pm can pick you up from the airport at 12:15pm. But you have to go back to airport on your own. See its website.

Senior travler ,is it best to take a cab or train to old town have 4 hours before boarding international flight

I agree. I covered a fair bit of the city (not the lake) in about 1 1/2 hours.

Yes, there is definitely enough time to take a train into the city. I recommend you just to walk around in the city, and enjoy the traditional old style architecture in this historical city.

If you like you may take a walk along the lake of Zurich, or walk along the shopping streets. It depends on your interests.