If the coming attractions for 4.5 are any indication, Peter remembers everything, but Olivia and Walter still do not. In 4.4, though, it's clear that Olivia is aware that Peter has been trying to contact her, via the ectoplasm that fleshed out (or in) in Reiden Lake, in our current time. This of course is the same Reiden Lake where Peter drowned as a boy as Walter was trying to take him back to our reality in the Fringe v2 we've been watching this season, and the same Reiden Lake that figured so crucially in Fringe v1 of Seasons 1-3.
Also of note is the Eternal Bald Observer who watches as Peter appears currently in the lake. This suggests that Peter's return is part of the EBO plan to deal with the "bleeding" of Peter into Walter and Olivia's minds, which we saw the EBO concerned about in the first episode this season. And this further suggests that what the EBOs have given, they can yank away - that is, pull Peter back out of our realities, again, if for some reason they deem that in their/ours/who-knows-whose best interests. And, to make matters even more fun, the EBOs don't necessarily all agree on the best courses of action, and to what extent their best interests are the same as ours - but this sort of multi-valent interest is clearly in our, the audience's, best interest (or at least, mine, as a viewer - I like this kind of stuff).
Speaking of Walter, he gets a vote a confidence from Olivia, who checks a box indicating that she does not want him returned to the wretched mental hospital. Regarding Walter's sanity, I'm always reminded of one of my favorite lines from Marshall McLuhan, that "the only people who have proof of their sanity are those who have been discharged from mental institutions" (from Take Today: The Executive as Dropout - see also my Digital McLuhan for more on McLuhan).
No Fringe on next week - World Series on Fox (I won't be watching, since Yankees won't be in it). And I may not get a chance to watch Fringe in two weeks, either, since I'll be in Europe then, giving Keynote Address about - yes - Marshall McLuhan, in Brussels and Copenhagen. But not to worry, I'll be DVRing and will post my review as early as possible in the week of October 31.
... Fringe 4.2: Better and Worse Selves ... Fringe 4.3: Sanity and Son