The fourth rift is, if it's similar to all the groups I've done it in, to the right (east) after the wall is removed. There's an area back there that's almost the opposite direction you need to go to get to the next boss (west then north). Every group skipped going that way thinking they could just keep going forward and then wanted to go scouring the map pulling trash to find it. It's funky because there are 2 different types of portals. One needed to spawn the boss that several have to be cleared and then a different set that's for the world quest. Both are marked on the minimap by yellow circles so they look the same. Most people don't understand why some of them disappear from the map which happens when the threshold for spawning the boss is hit on the first type of portal.