For the 5th night free you need all 5 nights in one reservation. Technically the hotel could join 2 reservations for that, but you can not rely on that.

Of course you can book a King for 5 nights and negotiate with the hotel later about getting a Twin.

Or you negotiate first and see what they can offer you.

You can also wait and see, if the availability will change to a Twin for 5 nights, but that could be risky as the availability for the 4 nights could go away as well.