You didnt mention what level these current toons are. You also didnt mention whether the fifth account is linked via raf.

You currently have only one melee and the rest are ranged. Also, all your toons and even the options are caster based damage. I like mixed groups like you have but i also like a mix of melee and ranged. I hate when a mob has a spell reflect and most of your team just has to stand there.

1. Keep your shaman. You could keep your shaman and switch it to enh instead of dropping it. If you prefer elememtal, i would keep it if the dps is satisfactory to you but add another melee dps.

2. Add melee dps. Cat druid. Cat druids do crazy damage without complicated macros or positioning.
OR
2b. Hunter. Hunters do direct damage that isnt spell based but you dont have to mess with click to move and interact with target.

Adding a druid lets you use leather drops, covering all armor types.

If you drop the shaman, and dont want in your face melee, i would add a boomkin for leather and a hunter for mail.

Basically, this is my opinion:
Keeping shaman?
Yes - staying elemental? - yes - add cat
.......................................no - add boomkin
Dropping shaman!!!
Want melee?
No - add hunter, add boomkin
Yes - add lock, add cat

My recommendations are based on being able to use every type of armor drop (since you dont mention RAF and it sounds like you will have one team) and having a mix of damage. If you dont care about not using leather drops and youre keeping the shaman and youre willing to box melee i would roll a frost or unholy DK and xfer an existing toon to the new acct.

Finally, i dont know what level you are but if you want to solo instances, holy is far better than disc. I tried disc and in the upper 70s i kept getting caught needing heals in the middle of smite spamming.