The fourth post and cross members go up.

All seemed to fit well at this end including the bracing post.

The other end was a different story. First the cross member was badly warped and second, the angled beam (not the bracing posts) was also warped at the end where it slotted into the vertical post. After a lot of bracing, forcing and eventually chiselling on the edge, I finally got this piece to go in with its bracing posts. However, not even shouting at it or hitting the cross member with a leafy branch would get it to fit. The bolts would just not line up. It was getting late so I retreated for the evening to consider my best option.