WATCH_DOGS

To complete this achievement you'll have to upgrade your RC car with the detonator. During Act 1 you'll receive the RC car. To acquire the detonator upgrade, you'll have to complete fifteen Militia contracts. You do not have to do fifteen different Militia contracts. I reached 15 by recycling through the same mission.

The first great place I found to kill 4 enemies at once is in Act 2:Mission 1 called "Bait". After you follow Tobias back to his hideout in "Hobotown", he'll lock himself in. After a cutscene enemy fixers show up in two vehicles. Each vehicle has four enemies in them. Instead of using the guns on the cameras, I quickly deployed the RC car and went towards the left vehicle. While the four enemies were crouched together behind a barrier, I drove behind them, got into the center of the group and detonated the car. 4 killed fixers, one 40G achievement.

Small video showing you how I got this after the campaign.

I highly suggest you do this during the campaign, but if you miss it, the video below shows the best method without restarting.

To unlock the C4 on your RC, make sure you have done 15 militia missions.

Follow the basic principles in the video with reinforcements and you'll be fine.

If you have already finished the campaign and don't want to restart, I found an easy mission to get this on.

In 'The Loop' there is a Fixer - Street Sweet and the objective is to download the 2 files. They rotate on the map so just be patient until this one pops up. It is to the right of the pawn show and in between two L-Train Stations (Madison/Wabash + Adams/Wabash)

Run to the first laptop on the ground, hide behind it and get your RC car ready and moving towards the gates, there will be 2 cars that come through from both sides. Wait for them to stop near each other and put the RC right next them both and detonate as quickly as possible. I got about 6 kills straight away. If you stuff up, let yourself get killed and it will restart.

I found an easiest way to get this achievement, in one of the 20 Chicago South Club contracts, could kill even more than 4 enemies, see the video.

I would like to suggest another alternative for getting this difficult achievement.

I found that a particular Street Sweep mission is well suited for getting this. It is located on the North East Island of the city pretty close to the center of the island. The mission takes place within a plaza that covers a full square block.

I snuck to the east side of the plaza and threw some lures into the plaza near two enemies standing next to each other. This will get a couple of other enemies moving around. Start driving your R/C car into the plaza in the direction of the enemies on the east side. They will only react a little to the R/C car and keep pressing the button to attract. The enemies will mull around for a minute or so but eventually will move close enough together that blowing up the R/C car will kill 4 guys.

Hopefully this works for you, I found it because I simply did not want to have to start the DLC over again.

When starting this mission place the RC car ready and then hack the laptop and move to the balcony. As soon as youve done that take control of the RC car and drive it into the enemy vehicles and blow it up.
Make sure you have the RC car explosive upgrade.