Step 1. Get 4 battle.net accounts and purchase 4 Diablo 3 digital copies of the game, and attach said "CD keys" to each of the 4 accounts, respectively.
Step 2. Get 4 full computer setups, 1 wireless keyboard and 1 wireless mouse, and set them to control the input commands of all 4.
Step 3. Put 1 installation of D3 on each computer.
Step 4. Attach all 4 to 1 internet connection via a router.
Step 5. Log in with all 4 accounts on each computer, and make the same class character on each.
Step 6. Profit.
:-D