I'm sure many have done this before - I've got a 12 prong Blanchard iron and unfortunately I went thorugh onto my stone and chipped a few of the teeth fairly badly.
Basically 8 of the teeth are fine - assume it might be best to just file down the 4 busted teeth and use this as an 8 prong iron from now on?
Either you file down the 4 teeth that got broken and make it an 8 tooth iron, or you file down all 12 tooth to the same level.
One thing to consider if you decide to file down only the 4 and use it as an 8 teeth, your iron will be out of balance, and you will have to adapt your grip and stiking angle accordingly. If you are using it as a real pricking iron and only going a quarter of the way and then using an awl, it shoudltn be much of a problem. But if you use it to penetrate all the way, i see this being a problem.