after you defeated Sin's mouth you go inside Sin. There you defeat Seymour for good and the part after that you can get an other Level 4 key sphere.
In that "rocky" area (in the beginning) you have a glymph sign on the wall to the right. If you touth it it will tell you how many fiends are left to kill. 10. after that you get an other of 10-12 and then one of 15. After those 3 are gone you see a chest and that where the other Level 4 key sphere is. <img src="/images/graemlins/smile.gif" alt="" />
