Kinect Joy Ride

First in addition to your main profile, you need to have or create 3 other profiles on your Xbox. For ease of writing these steps I'm going to refer to the profiles as follows:

"Main" = This is your profile that you want to get the achievements for.
"A" = New profile #1 or some other random profile you already have.
"B" = New profile #2 or some other random profile you already have.
"C" = New profile #3 or some other random profile you already have.

If you don't know how to add profiles, I'll walk you through the first one. Go to your dashboard and hit the Xbox guide button (glowing X in the middle of your controller) to bring up the menu. Now press to sign out of your main profile. Press again to bring up the sign in screen. Now press to create a new profile. Name it "A" (or whatever you like) and press . Just pick the default avatar option with , scroll down to "Done" and press again to accept. Now just repeat these steps for profiles "B" and "C".

Now on the main menu, just select the "Add Me" icon and choose your main gamertag. Now repeat 3 more times with 3 other local profiles and the achievement unlocks.

Really easy. Just sign in four separate profiles at once when you start the game and the achievement will unlock. Either play with three other friends or family members or sign in four yourself.