Use this Four Speedlite Adapter from Godox to quadruple your flash power. It allows you to attach four shoe-mount flashes securely to a separately purchased S-type bracket with a single knob. This allows you to use all four flash units at once for maximum power or adjust each flash individually for optimal and precise options in light intensity. See features for compatibility.
This is an update to my earlier 2 Star review. It turns out that using a wire cutter and a pair of pliers, I was able to nip off part of the Godox Adapter that was blocking and preventing my Canon 600EX-RT's from fully inserting into the adapter. Once I removed the material that was blocking, I was able to fully insert my 600's into the adapter and turn the Rotating Dial Lock to secure them into place. There's still plenty of rail left on the adapter and with the lock in place everything is secure. So, if you are comfortable with making this kind of modification to the adapter, it can actually work for Canon 600EX-RT flashes.
My Godox flashes do not mount to this unit, find out if your particular flash will mount to this unit before buying