I don't want to sound rude or obnoxious but I'd like to fit this just to be on the safe side.
EDIT:
The RS4 valve Increases the max fuel system pressure before it relieves it so better to have it on a stage 2 and 2+
@AJP Someone sent me this.
Like i said, stock handles 130 then opens to relieve pressure, but if you want to have a good amount of torque, you'll need the rs4 valve. When tuned, you may feel flat spots if you don't as most tunes request 130-133bar of fuel. Where as stock requests 127-130bar of fuel. Stock is really on the limit, but your tuner can decrease and tune around this for same peak Hp in the higher rev range.