u need to bribe the chimera brains 245,000 to make them drop up to 4 lv4 key spheres each. i think the minimum is 2.

to get money easy, fight mimics in omega dungeon with the gillionaire ability (rikku's celestial weapon has it) this will get u 100,000 gil per fight. or u can fight monsters in the arena and sell the equipment they drop.

Chimera brains also appear frequently behind the agency (between the agency and the path down to caladbolg). they appear on 2's making a possible 8 lv4 key spheres per battle (provided u have enough money, 490,000 per battle)

as dark messenger said, this is the only way to get lv4 key spheres after getting them from chests, bosses, etc