First off, a simple question like this should really be asked in the Simple Question thread that is stickied at the top of the page.
Well, since I am already here, I might as well answer your question. Your problem is that once you defeat the E4 and have the National Pokedex, Lorelei goes to Four Island (Icefall Cave) to try to stamp out TR. You have to go to Four Island and help her before she will return to the E4.