Cost-wise, you are MUCH better off with a longer ticket (7 days) versus two different tickets (3 and 4 day tickets).
That said, I am not sure you can buy a package with mismatched nights and ticket days (for example, you can buy a 4 day ticket if you are staying 3 nights, but not a longer ticket than that in the package)? Maybe you can, but quickly trying on the website I was unable? If that won't work, what I would do is make the room reservations as room-only. And then buy the appropriate ticket. Each room-only reservation will be one night deposit. And you'll pay upfront for the tickets.
Of course, play around with the website to see how the pricing varies.
And I love split stays. Just have a solid plan for the day(s) you are room-less.