I’m not sure why you would need to buy a 7 day ticket for four parks days. They dining plan is tied to your length of stay not the number of days on your admission ticket. You can not replace park hopper by using more than one day’s admission on a single day.
Disney will only let you into one park per day on a non hopper ticket – period. Unless you will actually be there for 7 days and want to have the option of going to the parks on nonpark days, it makes no sense to get extra days. You can add park hopper at the park if you want to wait and see if you want to hop, but it is $45 per ticket regardless whether you buy it in advance or at the park.