I'm not sure about outside the airport but I don't think 4h15 is enough time to get anywhere as just as you've arrived you'd have to return. Night-time is prime time around MIA as that's when most of the south american flights depart and it's sure to be a zoo so you might not want to chance it, but others would know better for sure. As far as IN the airport, I had a great time just people watching. Arrive hungry and head over to La Carreta near the entrance to the D councourse for a Cubano sandwich and fries... not healthy but YUMMY. They have lots of other much better for you options as well. I had key lime pie too which was awesome... they don't exactly go together but key lime pie is my favorite dessert of all time so I couldn't resist. Note that none, and I mean NONE of this was on my diet.