If you want to PvP/Arena, 4 shamans is fine. 4 Shamans will run normal mode instances fine as well. If you want to do heroics, go with a Pally healer or a 5th DPS of some sort (not suprisingly, a shaman works well).

Taurens and orcs both have their bonuses. I personally prefer orcs since the avatar is more compact. A tauren would allow you to hide them all behind the main, though, if you made the new guys orcs.

For easiest leveling, powerlevel the three new guys to 40 using instance runs with a L70. Shaman would work fine here but a mage or AoE melee class will do better. Quest with the three of them from L40-58, using your main shaman to help out if needed. Put him ungrouped and on follow to discourage gankers if you're on a pvp server-- he can help with tougher quests also if needed this way. Go to outlands at L58, quest for the good Hellfire gear, then add your fourth L70 shaman when the new guys get to L61-62 or so and farm Ramparts, Blood Furnace, Slave Pens, Mana Tombs, etc. Come out when you need to complete key quests (lrn2wowheadsearch) for elemental gear upgrades or when you get bored of instancing. Can quest more if you're not on a PvP server, but you'll level faster by avoiding ganking and staying in instances if you're on a PvP realm.

Only reason to get 5 accounts is to do heroic content, IMO. 4 shamans is all you need for arena and probably has an advantage in all BGs except for AV, where the difference between 4 and 5 is not super huge. If you're making the plunge to 5, might as well do it right and grab a pally instead of another shaman.

Sorry for the terse reply, but your questions left a lot to be desired. Most of this has been repeated multiple times in very easy-to-search-for messages...

Good luck,