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.
Location: In a cottage waiting for my mansion over the hilltop!
Thank you Shane for helping out!
Next time you have a question, do as Shane says and ask it in Simple Questions. The link to the OVERLY helpful thread is in my sig and in the group of stickies near the top of the FR/LG (GBA) thread listing.