4. If anyone other than the player kills the boss, they don't drop the special finger. In that case, you need to go back to Regulator HQ and reset the camp. It's especially annoying if you have companions as they're always kill-stealing.
This is also an issue in killing bad guys in general when you're a Regulator - if a companion kills a bad guy, they don't drop a finger. You get XP, but no finger to turn in to Sonora. Also, fingers are stupid heavy. I use an NV weight mod that allows you to change the weight of everything just so I could change the weight of fingers. Seriously, before changing the weight, I'd often have 20 to 30 pounds of fingers before getting around to going back to Regulator HQ.