If you're at 3/4 and can't fine the last one, check the southernmost point of the instance/quest area, due east of Zuraal. My group missed it since it's not very close to the bosses.
You have to do this as a group. If you aren't together, and someone clicks one, it doesn't count if you're not near it. Then you have to run the dungeon again to get that last one.
You need to close 4 Dark Fissure and kill last boss
. If you want to do it quick go HC.
See My Video if need it -
Seat of the Triumvirate: Dark Fissures Video
[Seat of the Triumvirate: Dark Fissures]
[Dark Fissure closed]
Argussian Reach Reputation Guide