No way you can do Rome justice in 4 days, let alone 2. A day at Vatican City, a day at the Forum/Palatine/Capitoline/Coliseum, a day in the downtown core hitting the Spanish Steps, Pantheon, Piazza Navona, Trevi Fountain, a day meandering around bohemian Trestevere, a day of shopping Via del Corso and Via Condotti...I'm just getting started. And don't forget that you will probably spend an inordinate amount of time eating (a good thing given the quality of food!)

I would definately recommend chosing one or the other. Keep a close eye on your wallet and steer clear of Roman taxi drivers if at all possible. They'll take you for a ride...in more ways than one.