I have 4 shadow priests myself. I can tell you that PvE wise you can do almost any non heroic.

Without a 5th party member, I can kill the 1st boss of shadow labs..you just need to plan ur fades properly to switch out the aggro which is really easy to do because each of your priests generate an insane amount of aggro. I can clear the 2nd room...cant kill the ogre...because 4 shadow priests turning on each other is not pretty. I can also kill the 3rd boss with just the 4 priests. I can not kill murmur...though i havent tried with a 5th.

Im currently using the frozen shadoweave set, with the extra pieces being the blue pvp set with the the pvp epic items and all +12stamina gems. I have about 700spelldmg and 12k hp buffed. I had about 500spelldmg and 9k hp buffed when I did the shadow lab runs.