Yes, Oman is dull, no doubt. But it's a great place to visit. The Muttrah souq and corniche area are a quaint and friendly slice of Gulf Arab culture, with a lot of historical multicultural influences (British, Indian) built it. The people are by far the friendliest in the Gulf region in my experience, and the place has an authenticity to it that the UAE and Qatar lack to an extent.

The country has a few true destination hotels (Chedi, Shangri-La), though the main five-star chains (InterContinental, Hyatt, though I am not sure about the newly renovated Ritz-Carlton Al-Bustan Palace) are a bit dated.

There are passable Arabic and Indian restaurants, and little nightlife as others have pointed out.

There are some nice geographical features outside Muscat, e.g., up the coast toward the UAE and down toward Salalah, which you could include in a four-day trip, and some forts and whatnot.

If you're only going to visit the Gulf once in your life, then I would recommend Dubai. But Oman is a nice secondary destination.