I say take pops to a big bike shop so he can actually touch the bikes and sit on 'em and stuff. See what he likes the look/feel of. 4 miles one way means he can ride just about anything he wants and still have a good time.

Maybe the old man would groove on a retro lookin' cruiser bike with some gears on it?
Maybe Dad thinks a mountain bike looks like peak fun?
Maybe he likes the looks and performance of a road bike best?
Flat bar roadbike hybrid?

After picking a style, you can narrow down your craigslist options. Pick a budget. Then show him a bunch of choices. When does your town have the annual police bike auction?

I think it's cool that you and your Dad are into the commute thing. My Pops is an old time bike racer. Commuting just didn't register for him until last week! He can't believe he didn't start bike commuting sooner.