I doubt it's Locke.
Keep in mind here one pretty significant fact. When Jack visits the coffin, it's before he tells Kate they need to go back to the Island. We all know that's not the end of the story which means the events we see are probably a couple months into Season 5.
If Season 4 is about how the Oceanic six get off the island - which is what the previews and news sites seem to indicate, then Season 5-6 will be getting back and resolving all the other storylines involved.
If Locke is in the Coffin, that means his story is over at the end of the this season or early next at the latest which means no Locke for basically two seasons. I really don't see that happening.
So, who's expendable for the last 1 1/2 - 2 seasons and would ellicit the types of responses we saw from Jack and Kate?
- Michael? Clearly. He gets my vote even though he's the obvious one.
- Sawyer? perhaps, but very unlikely given the popularity of the character.
- Desmond? Doesn't make any sense at all.
- Ben? I doubt it based on his dealings with Sayid and the popularity of the character.
- Locke? I don't see them killing him or making him leave the island at least until the last season, if at all.
No one else really comes to mind so given those choices and the fact that the events take place after this season but well before the end of the series, Michael gets my vote.