What are Best things to see in Rome in 4 days

All good guide books have a list of the "must see items" in Rome. There are a number of big ones. Second, Pompeii is south of Naples. It can be done fairly easily as a day trip from Rome BUT it will be long trip with about six hours of trains (3 hrs each way). Pompeii is always worth it but I personally would not do a day trip. I would save it for a more convenient time but a lot of tourist do it.

I really recommend going to the top of the Victor Emmanuel monument for the best view of Rome and it's ancient sites.

kmspace:

If it were me, I'd skip Pompeii and do the following (in Rome) not in this order or necessarily all of it but I'd squeeze in as much as I could...

Vatican Museum and St Peter's
Galleria Borghese
National Museum

Piazza Navona
Piazza Fiore

Trevi Fountain
Coliseum
Roman Forum
Pantheon
Victor Emmanuel Monument
Trajans Market

Ostia Antica is a good alternative to Pompeii and it is only minutes away by urban train from the Piramide metro station.

Rick Steves has an excellent planning guide for Rome in his book. The plan is here on this very website. Do you have the book? The Guide to Rome? It will answer questions such as how to get to Pompeii on a day trip, if you choose to do it. I agree with Frank: It is not a day trip I would take.

I agree it sounds like a super long day.. but frankly,, next time I am in Rome I will do it. I have been to Rome three times now and never gone.. and really want to.. and still regret not doing it..
I however have already seen many of the first timers must sees..

So.. I guess for me it would be.. are you ever planning on returning to Rome.. I personally always assume I could get back to a place.. but some folks don't look at things that way.

As the Parthenon is in Athens, Greece, it is not recommended as a day trip from Rome.

As the Parthenon is in Athens, Greece, it is not recommended as a day
trip from Rome.

I'm pretty sure kmspace meant the Pantheon. Pompeii is definitely 'worth' exploring but I wouldn't do it on a 4-day, first-timer's trip to Rome - which in actuality will probably be closer to 3.5 days if that's your entry point to Italy or you're coming from another city. You'll barely scratch the surface of Rome in that amount of time, and Pompeii is a very large site quite some distance away so it's a long day. It's better included on itineraries which include stays in the Naples/Sorrento area.

Ostia Antica is a great substitute when staying in Rome as it's much closer.

Definitely agree with what everyone has suggested seeing. I would recommend getting a 2-3 day pass for the "Hop-On-Hop-Off" Bus tours. With that, you can easily do the Roman Colosseum and Forum along with the Pantheon, Trevi Fountain, etc. all in one day. It was about 24 Euros and made it super easy for us to get off at each stop, see what we wanted to see, and hop back on to go to the next site.

I purchased a Hop on Hop Off bus ticket last October. The red busses seemed to have more busses than anyone else. I purchased on the green bus (different companies - sorry I cannot remember the names). I waited for over an hour for the green bus to pick up near the Victor Emmanuel Monument. You never know about breakdowns and things going on that you cannot see, but I started realizing I could walk some places faster than wait for the bus. If you get the Roma pass it includes admittance to 2 sites (The Coliseum, Roman Forum and Palatine Hill together all count as 1 admittance) and a 3 day pass for city busses and the Metro. With that pass you can navigate the city without walking.

On the other hand, I did use the Hop on Hop Off bus to just ride around the city in the evening. If you can get a spot on the top of the bus, I find that to be a very fun way to spend an hour. I just rode the full loop around.

I know that Rick Steves suggest that with 4 people a taxi is economical in Rome.

The fastest Rome-Naples trains take 1h 10min each way, and the local Circumvesuviana train Naples-Pompei Scavi takes 40 min each way, so total travel time can be a bit less than Frank's estimate, even with connection time. The last return of the fast train departs Naples at 19:30, and slower options after that.

I'll respectfully provide a different viewpoint on the Hohos and Roma Passes:
They've (the hohos) been getting pretty bad reviews for some time now, and there have been a lot of complaints about dirty buses, broken equipment and rude personnel. Unless one has a mobility problem, I would pass on those.

The passes work in your favor only if you can cram in enough of what they cover while they're active, and use them often enough for public transit. Don't let the promise of "free attractions" throw you: you have to do the math as it doesn't always work in everyone's favor. Tickets for attractions with long lines - such as the Vatican museums, which the Roma Pass does not cover - may easily be purchased in advance.

Half the fun of Rome is seeing it from your feet. Your shoes can get you into tiny streets and up stairways where vehicles can't go, and you can often do it more efficiently than waiting for a bus that doesn't show up or passes by a stop because it's too full. If you plan your sightseeing by area, you don't need to be running from one end to the other, either. Central Rome is also not really all that enormous.

Time may also be managed by seeing the attractions with visiting hours during the day, and the walk-bys which don't for the evenings. For instance, Trevi and some of the piazzas are very pretty at night when they're all lit up.