WalMart Customer Shoots Unarmed Marine At Checkout For Touching His Cart: Said Marine's Cursing Caused Him to Fear for His Life
My first thought is that the four friends either were so well behaved that you didn't notice them until you were right upon them, meaning they weren't being disruptive and you were the instigator OR you did notice their boorish behavior before heading down the adjacent self-checkout line and you weren't in fear then and decided to provoke them uninstigated OR you noticed, you feared them, and then instead of heading a lane over you escalated by then confronting them. Either they weren't a threat or you chose to place yourself intentionally in harm's way in my opinion.
Once it got to the point of a guy touching your cart on one side and you feeling like things were escalating, what could you do? It seems to me that you could turn around and walk away, or back away, just fine. There are a lot of potential reactions from the guy, but it seems the most likely would be to talk smack with his friends without moving. Had you not even tried diffusion techniques or to get out of the situation when you initially thought they were safe enough to approach and correct, meaning they weren't doing anything different except touching your cart, I wouldn't have much sympathy were I on your jury.
I recommend some situational awareness from now on. That would have avoided the entire encounter.