I'd recommend NYC over Boston. There's lots to do for 4 days and it's a very easy city to get around. It has so many of the country's iconic landmarks such as Times Square, Empire State Building, Statue of Liberty, film locations for numerous US movies and tv shows, as well as great shopping, restaurants, museums, etc.. So it's really an ideal location for a first trip to the US. Boston is ok but is quite boring in comparison to NYC.