Melissa, Michael and Walt WILL be back in Season 4, so you could be very right!
BTW, have you guys seen the sneak peek for 4x01 - The Beginning Of The End? Jack and Hurley are talking through the walkies and Jack's like "You'd better pack your bags." Oh man I can't wait for the next season! I'm so friggin' psyched.
Oh, Guess who else will be back? Highlight if you want to know:
LIBBY!! She was like my favorite character! She'll be back in flashbacks.