If the route is fairly flat, practically ANY bike will work. If it's hilly, you need gears, but just about any bike with gears will work. In other words, in terms of making 4 miles a day, it won't much matter because they all qualify. Get something fun to ride; that'll motivate you to ride more.
On the disk brake- you probably don't really need it. I've used a crappy mountain bike, and the brakes had problems, but that's because it was a crappy bike, not because rim brakes are bad.
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"be careful this rando stuff is addictive and dan's the 'pusher'."