Four days is a nice amount of time, although you could accomplish most of that list in a single long day if you wanted; if you take Metro A to Spagna (the Spanish Steps) you can see those and then take a nice walk down Via del Corso to the Capitoline and the Forum (maybe a twenty or thirty minute walk, but always lively; I would spend another whole day there seeing the Musei Capitolini myself) and then a ten minute down Via dei Fori Imperiali to the Colosseum and you can hop back on Metro B from there...