My main question is why only 4? Are you limited by computer power? There are guides to help reduce comp requirements.
My followup question, what level are you? Shaman will be a weaker healer until higher level, which may be why you might be running a healer merc?
If you want a melee group, what people have said holds true. The holy trinity - Tank / Heal / CC.
Right now, in one of my groups I am running SK, CLR, BRD, BST, ROG, BER. And now that i'm getting higher levels (80ish) the melee are absolutely melting mobs. My BST did take longer to come around, but I hear 100+ is when they start to shine. Be sure you are keeping the poison up on your rogue, it's insane at lower levels.
If I were you, I would add 2 more box chars, so you could be SK, SHM, BRD, ROG, and then add BER/BER. Since you have the SHM, I wouldn't worry about the BST. (I'm currently leveling up another group which will have a shm and ber to swap out later so i can have that make up)