I may be on the opposite end of this discussion but when I go down for a week we always get a seven day park hopper pass. We've been to WDW many times so having a park hopper is a must. We try to arrive early that first day so we hit the parks right away. We have special things to see in different parks so we tend to go one place during the day and one place at night. Sometimes, we may hit a park in the AM, relax at the hotel in the afternoon and then either back to the same park or another park that evening. If it's your first trip or two, going without the park hopper may be a good savings. You just want to plan your days a little more to try to avoid crowds. As far as the number of days, the difference between a 4-day and a 7-day is only $3/person using the park hopper option or $8/person with a straight one day/one park ticket.
Since the price is so minimal between a 4 and a 7-day ticket, I always find it better to purchase the 7-day and then I can go into a park anytime I want to.