When can I stretch my ears to an 8g? then 6g.. and possibly 4g after.?

It all depends on your pain tolerance. Your ears, as you have probably figured out, will swell and be very irritated, but it is possible. However you need to make sure you don't rip your ear. And I don't mean like literally rip your ear open. On the inside the skin can rip when pushed too far. It hurts until it heals but it won't ruin it. But from then on that ear might be more prone to ripping. You need to really be cautious and use sterilization. Good luck!