According to Marlene King, executive producer of Pretty Little Liars, here are some spoilers for the season 4:
- we will meet Toby's mom (I know it's been said in the Pilot that she died, so we don't really know if it will be a flashback or something like that)
- Ali will appear more often (though, of course, it's not yet confirmed if she will be alive, a ghost, a twin, in a dream, flashback or whatever :p)
- Mona will stick around
AAAND, the most important spoiler (for me, at least):
- we will be introduced to a several new characters in Rosewood, some that are wanted from the books by fans for a quite some time :D