Yes, the dining plan is allocated by nights, not days. If you stay 4 nights and 5 days you will receive credits based on 4 nights, not 5 days.
Second, even on a DVC reservation there is no longer any way to do this without booking separate room reservations. For awhile last year DVC allowed members to "segment" their reservations so they could stay in the same room and use different dining plans or a partial dining plan, but that has been discontinued.