If I was this girl's mother, I would be sitting her down and making her look at some pictures of facial burns, and scarring following burns - to bring home to her once and for all the possible consequences of playing with fire. I'd also be making her come and apologise in person to the OP's son.
Ds3 has a bit of a habit of playing with fire - he hasn't tried the aerosol/lighter combo, but has played with matches a few times, and once (aged about 8) scrunched up a sheet of paper and put it down the side of the bulb in his (lit) bedside lamp - to see what happened. It started to smoulder, and he panicked as he heard dh coming upstairs, so threw the smouldering paper behind his pc tower, where his curtain hung to the floor - then denied altogether the presence of smoke in his room when dh asked. Luckily dh was not taken in by this and investigated, otherwise the curtain could have caught fire.
I have read ds3 some pretty graphic lectures, and he has been told that if he ever plays with fire again, I shall be contacting the local fire station and taking him for a visit, so that they can put the fear of god into him.
And for those who have said 'it was an accident' - I have to disagree. It was an accident that the OP's ds got hurt, but it wasn't an accident that the flame got sprayed, and it was entirely foreseeable that there could have been someone behind her, so more of an 'on purpose' than an accident in my book.
I'd have called the police in the OP's position - 4 days suspension is entirely inadequate punishment. FGS, the bully who pulled down ds2's PE shorts during a lesson got suspended for 3 days! I felt that was a proportionate punishment - it was a horrible incident, but ds2 felt happy at the way it was dealt with (it gave him confidence in the system at his school, which is why I believe it to be a proportionate punishment) - but to say that something this reckless and dangerous, that caused physical damage and psychological harm to a child deserves only 1 more day's suspension than what happened to ds?? Ridiculous!