you can get a ticket for any length of time with the hopper option, and im pretty sure you can do a 4 day pass for only a few dollars more than the cost of a 3 day pass, so if you think you might want 4 park days, i would spend the extra $5 and get the 4 day pass.
whether or not to get the park hopper option depends on how old the youngest members of your group are. i have 2 kids under 3yo, so naps are essential. they need to nap in their beds/pack n plays, they cant nap in the stroller, so we have to go back to the room mid-day. then in the evening we go back to the parks. sometimes it is a different park than we went to in the morning, so for us park hopping is a must. its nice to be able to do animal kingdom in the morning and magic kingdom at night, since AK closes so early anyway. or go to epcot and see the characters in the morning and MK at night, or whatever. but if you have older kids who dont need naps and who can spend a full day in the parks, then i wouldnt get the hopper option.