I would probably grab a cab to Old San Juan and do a little walk/window shopping around the old streets. And only for a couple of hours. But we spent 3 days there before our cruise in '03, and got to see all the forts that DH wanted to explore.

If you've never been there before, you might not be comfortable doing this, since you would not be familiar with the streets, etc. I recommend looking at the ship's excursions and seeing what sounds interesting. With only 4 hours, you do not want to take a chance that an outside excursion company would not get you back in time.

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