its because you are 4 adults or kids aged 7 or older (ie no free kids in your party).
If you want to stay onsite I'd book one adult in on the package (around £280 for your 4 nights, including park tickets for that one person) then get annual passes for the other 3 of you, and add those 3 names to the room at check in. You don't pay anything extra to do this, just local tax which is no more than 2 euros per night booked, per extra person added to the room.