So the 40 million per launch includes the cost of the four payloads too right? Presumably the 5th payload is already available from another source?
No, the 40 million is only launch. Payloads are in the separate contract of 2.78 billion RMB or 430 million USD.
Yes, the 5th payload is presumed to have been sourced.
No, it is likely that the 5th is from the same manufacture (5th academy) and same family (中星) but unknown series. China's space industry remains the way that different entities specialize in different types of products, I don't think any other entity is capable in making similar satellite in this specific (communication) and high value category.