I've never had Four Loko but Caffeine + alcohol = bad times.
I've had two bad experiences with caffeine and alcohol. The first was with Rockstar Vodkas and a lot of pukage.
The second time was fucked up though. It was the second last day of my internship. We started drinking at 3pm at the office. One of the perks of the job was free alcohol...most of the standard types of beer, wine, and liquor...and so someone started mixing me some Red Bull vodkas.
By 7pm, we were at Duff's eating wings (and drinking more - I still felt pretty good at this point and I think that was cause of the caffeine).
By 10pm, we were at some other bar but I don't remember that part of the night at all.
By 12:00am, I apparently decided to go get some more money. I found out later that I actually passed out at a bank machine nearby. Some person walking by found my cellphone in my pocket and called the last number dialled, which was one of my former roommates (who lived downtown and pretty close by) who took a taxi to grab me. But by the time he got there, an ambulance had already come by.
I don't really have much recollection of what happened in the hospital though I think they mainly IVed me. But I was told by the doctor that my blood-alcohol level was 0.36(!)
Yeah I got a slow clap the next day rolling into the office. At least it did ensure I was one of their most memorable interns.
All kidding aside though, I haven't touched caffeine when drinking alcohol since then. Hell, I don't even drink energy drinks anymore. I honestly think the caffeine made me feel far less drunk than I was in the first few hours which led me to drink faster than I normally would.