The Great NorthernWall Bonus Mission

I have found three out of four items for the spirit.
Can anyone help me by telling me where I can find the fourth piece?

Its always around the perimeter of the Spirit, they can be found in "wreckage" containers. It took me and my friend a while to find the last piece, but we got it.

What bonus mission is this? I've gotten over the wall in pre-sear Ascalon, fought most of the Charr but didnt have time to kill them all. Is it an actual quest/mission or do you just find the spirit items?

It is a side quest from a ghost. The parts of armour are fairly close to him. Just run around in a big circle following the path and use ALT. You will find the wreckages.

so the ghost is also in the northlands?

It is the Secondary Mission in the first mission post-searing that they are talking about.