HighTone said:depends on your budget, getting a 5 or 6 stops is good. otherwise get a 3 stop minimally.
For some reason, I find this quite funny too.
You use what you need. To get 4 stops u can use either a single 4 stops filter or combine one 3 stops and one 1 stop filter (or two 2 stops but that's unlikely). Whether u want to stack or use single, it's really up to you.