Let's see here...
1) Jon is at the Wall and is LC, he'll be becoming a more serious character (as though that it was possible to be more serious than he already was)
2) Stoneheart will be the big reveal at the end of the season. In the meantime stories about tBwB will be told
3) In Dorne I can see things picking up nicely with Oberyn's death toward the end of the season.
4) Cersei will devolve into a paranoid schizophrenic and become super power hungry in King's Landing.
5) Jaime will be in shock, which should further propel his redemption arc in the Riverlands
6) Brienne will likely have spent some time with Jamie in KL in season 4 (where she will get her blue armor). I see her role as tricky, to use Brienne the next books are dipped into and I'm not sure how to divide her storyline.
7) Sansa will be in the Vale taking care of Robert and dealing with LF. I also expect Lysa to die in S4, and after we see her falling to her doom and LF's quip it changes to Stoneheart's first strike.
8) Bran will be continuing toward Bloodraven, although not sure if Cold Hands will be incorporated. Bloodraven and the CotF will not be revealed until late S4 to early S5 with that fight just before the cave.
9) Arya- journey to Bravvos and a bit of FM training.
10) Sam- In S1 Sam mentioned that he always wanted to be a wizard. Gilly calls him a wizard for his reading ability. I think this foreshadows Sam learning some magical ability at the Citadel. Additionally I see his S5 storyline starting in Bravvos.
11) Mel- don't expect to see her early in S4, maybe they show the sacrifice for the winds. By the end of S4 I expect the Battle of the Wall to have taken place and her to be establishing herself as Jon's annoyance.
12) Dany- sit in a pyramid, become a more grey character and upset Hodors, hopefully the Jorah betrayal comes up. By S5 she'll probably be ready to marry Hizdar.
13) Barristan- will council Dany more with Jorah presumably gone.
14) Quentyn- will likely make no appearance at this rate. But if he does it will be with an introduction at the end of S4 and appearing S5 with our first look at Volantis.
15) Kingsmoot- may show Balon watching Asha leave to get Theon and then dying while pensively crossing a bridge with some random servant passing through the shot that may be Jaqen. The Kingsmoot will not happen until later in S4. Or it may be done away with altogether with Euron just sweeping in to claim it and everyone grumbles but allow him.
16) Rickon- will have to leave some trail for Davos to pick up on later.
17) Theon- help with surrender of Moat Cailin. And at Winterfell with the procession of lords enroute.
18) Faegon & company- definite S5 appearance. Who would you want cast for that role? To me it's a definite unknown coming in.
I think that covers most of what is to happen in S4/S5. My guess is that it will be slow gong for S5 with Tyrion in Essos, Faegon & Company, Dany marrying Hizdar, Jorah turning up in brothel, Cersei's melt, Jamie running around the Riverlands, The Ironborn united under Euron, Loras being at d-stone, Myrcella in Dorne and DARKSTAR, Jon dealing with Janos/Stannis nod (
) and acclimating the Wildlings to south of the wall with Val (please don't write her off), Sam gone to Bravvos, Arya in bravvos, introduction of Manderly.
shite there is a lot going on. Not at all sure how they'll deal with that. Some major GRRM consulting will be necessary.