Hi Rachel!
I just answered another question about this very same thing concerning the
Walt Disney World Tickets. It can be very confusing for many of us and things even change constantly about ticket rules and regulations so it is hard to keep up sometimes.
According to my phone call to Disney about this issue for guests needing only a part of their vacation ticket package for park hopping this is what you would need to do.
If you buy a four day ticket package then you would need to use the first day ticket as is. Once that is done you can then go and add park hopper option to the rest of your tickets. You cannot add only one day for park hopper to be used on any of the days. If you only need one day then it would have to be at the end of the four days as once you add the option it has to go to all remaining tickets.
Your other option is to buy a completely separate ticket that has the park hopping option on it and not part of a multi day package. When you do this make sure the ticket is in place so you are using the correct ticket on the correct day.
If you have questions when you arrive it is best to head to a Guest Relations location either at the parks or Disney Springs. They will know exactly how you will need to handle your situation and will help you get things straight so you will have a magical vacation!