In the longer, missile bearing version, the pilot only controls the fuel dump. There is a master dump switch, then a choice of two switches for dumping from either the front group or rear group of fuel cells, or both.
It comes out near the top of the fin and can be ceased at any time and will cease automatically if you get down to 300kg in either group (but you've really f****d it then unless you're about to land!). The switches are all on the front right console just below the warning panel and I believe the GR4 is exactly the same.
Why could you possibly want to know this? Wouldn't you rather have some secret weapons info?
Ray