As everyone stated, supplements are ones personal preference and all are not regulated by the FDA so any research done is done independently by that company. There are a few supplement sites that you can Google that offer reviews on supplements but be wary that each supplement affects everyone differently.
I’m not recommended any but there are a few different pre-workout supplements that contain no fillers. I would suggest looking independently into the ingredients that are in C4 and any other supplement you are interested in.
Hope this helps