You need a healer for sure and Teresa inquisitors is doing better on pvp
And if you not gonna replace anyone on your 4 man squad, you might want to build up Kiera's power to see how a demon do demon things cuz she got buffed this patch
Generally speaking, if you are lacking firepower then use the elementalist, but if your frontline die too fast then grab the inquisitor.
You have to test out whats the best combo for your team. You can do this by challenging people from your guild and friend list in the arena with different formation.