Can't see Chelsea dropping out of the top4, they've got a lot of tough fixtures out of the way and are starting to get players fit again, plus they have the financial muscle to bolster the squad in January if desired. Realistically looking at the table you'd have to say that of the chasing pack only Spurs have the players to compete with them, yet their failures mean that not only are they behind Chelsea on points, they also have a vastly inferior goal difference.
That may sound a little harsh on Spurs but the reality is if you want to be a top 4 side then after 18 games you'd be targetting something above 30pts and +4 GD.
(finding midseason tables is a bit tricky but as a guide 2 years ago the team in 4th spot at Christmas had 32pts and +17 from only 16 games).