This is useful but really specific. In order to get a 4th crit from your combustion skill, you must:
1) Count the number of crits you have currently used.
2) Have Fire Blast up.
3) Have a bit of luck.
Now, from what I can tell your crit roll is done at the time you cast your spell and not at the time the spell hits the mob. What you need to do is after your second crit use fireball (assuming your % to crit is at least 60 or so) and right after (maybe even in the split second before the spell cast when you can queue a new spell) your spell is cast cast fireball. If you were lucky they both were crits.
Not extremely useful, but something I noticed while I was leveling a new mage char. I don't know if its actually a glitch (that until the fireball hits the combustion buff remains active) or if I've been lucky. But from my own trial and error I've done this successfully quite a few times.
Note: This only works if the fireball was rolled a crit, then the fire blast, too, will crit.