Must be done on the Doggie Bag DLC maps.
I had many difficulties trying the above method due to the difficulty in escaping the final round and came accross the following.
In the first round the player going for the achievement stays back. All other players go and collect some loot (approx 100,000 each, or more as long as the combined total is below 1,000,000) they then proceed and immediately die and become police.
Upon respawn as police they proceed to collect the loot which is left where they previously died, return to the start and kill the other player thus ending the round and sharing the collected loot.
On the 2nd round everyone just dies immediately.
This will begin the final round with you in last place. Similar to the 1st round 1 person needs to stay back (this time not the person gaining the achievement) and everyone else needs to collect all the loot (4,000,000 in total). Again these players proceed and get killed straight away. The person going for the achievement then collects all the loot so therefore carrying 4,000,000 and all other players return to the beginning of the level where the other player then proceeds to kill the players who spawned as police. Now all you have to do is go back to the start carrying all your loot, kill this player and the achievement will unlock.
This achievement unlocked for all 4 of us in my session and took no time at all compared to our previous attempts where we tried to escape in the final round.
Hope this helps.