Looking at the club that are still well within reach of 4th, I think it will be Arsenal realistically. But...
If Newcastle pick up their early season form,they will certainly be within touching distance. We are still only a win away from going 4th as things stand. With Cabaye and Tiote back in the team, our midfield is as strong as anyones. The past three games without Cabaye we look disjointed, no threat from midfield. Since Tiote left for ACN, we have had no combative player, which has meant the area of the field was ripe for the picking. The protection Tiote gives the back 4 helps(look at the games he hasn't played, compared to the games he has played, the stats are clear). Add in two top class goalscorers, and you would be wrong to rule out Newcastle.
But to be fair, give me a top 7 placing and I go home happy in May.