I'm not sure about a car for just 4 days. Is this your first Ireland trip? If so, you're going to stay pretty well occupied in Dublin. You can take a day trip by train down to Waterford and then a short cab ride out to Waterford Crystal, which does a very good tour.

If you were to drive anywhere, I'd head to the west coast, particularly to Galway and The Burren area, or even Killarney and the Ring of Kerry. Touristy, but scenic. I've even been over to the Aran Isles, but you would not have enough time for that. You can easily get out here from Dublin in 3-5 hours (it's not that big a country). Dingle or Doolin (known for pub music at night, and near the Cliffs of Moher) are also worth visiting.

I've driven in Ireland before, and the one thing I didn't care for was the drive in and out of Dublin. Once you're out in the country though, or out west, it's fun to drive, and quite scenic.