When you beat the e4(which u hav already done ) u should go to Mt.ember and to the place where the 2 rockets are standing....this time they will be blocking a cave instead of doing nothin..(lazy fella's). defeat them and get entrance to the cave....then get the sapphire plate from the cave but a scientist will steal it from u...you have to go to island 5 then..to the TR HQ. there battle and get the plate...done!!
Cerulean Cave: pass the nugget bridge...on the right side ull find a narrow lane of grass with a picnicker standing in it...go down and youll come to a lake..surf in it and keep going..youll reach cerulean cave's entrance in no time!!!