Bard does great for pulling and will help with ADSP. I was tempted to say shaman or another mage however you do need to solve the pulling issue unless your warrior is hell on wheel equipment wise...then pull em all
I would go for an enchanter, but bard would work too. No other options would make much sense. Enchanter would provide better ADPS for your mage/wiz. Bard would provide better ADPS for your warrior. Bard would provide selo speed and track. Enchanter would provide better CC options.
Bard. While the enchanter would probably be better if played optimally, I can’t imagine that you would be able to play it as well as you could a bard for the 4th box. Bard can achieve a significant amount of ADPS through /melody and you can pull very quickly by taking advantage of fading memories. 5min selo’s speed AA is also gamechanging — I couldn’t imagine playing the game without bardspeed at this point. It would all feel so slow!
Best part about this group is warr, wiz & mage are extremely macro friendly set and forget type boxes. I def recommend the enchanter also.
So I am assuming your running 3 toons with 3 healer mercs. My question to you is; what content are you doing now? What content do you want to be doing? Adding another toon will take healing away from the group. I do not know if you need that healing. Because of what other toons you have I do agree with enchanter, however, I am curious on your thoughts of why you need a 4th.