I'm having the same problem .. stuck in WT2 and I'm not seeing any workaround or fix. Each time that I complete the strongholds and progress into WT3 or WT4, it reverts back to WT2 as soon as I leave that group. Following this thread for help
Try doing the stronhold in your session not in your friend's session...i had same problem with a friend with got to T2 on my map...my friend go on his map...bang T1. We had to redo the stronghold once again but this time on his map...so i guess same can happen if you are 4...