In the current version of Carcassonne, two-tile cities are worth 4 points.
But this is not the only rule that has changed between editions - farmer scoring has changed as well. Below are the scoring differences (copied from here):
First Edition Scoring
If a farmer can walk along the field to a city, he is connected to it.
It can be fifty tiles away, or on the same tile. As long as unbroken
path goes to the city then he gets counted.
For each completed city, the player with the most farmers touching it
gets four points. Tied players all get four points, each.
Note that two-tile cities are two points for a knight when completed.
Second Edition Scoring
Count the number of farmers in that field. The player with the most
pawns inside the field will score for that field. If many players tie
for majority of pawns, they all receive the full points.
A field will score 3 points for each completed city adjacent to the
field. A city is adjacent to the field when part of the city walls are
used to define the boundary of the field (i.e., next to the field).
A player can score for a single city one time, and only one time... no
matter how many fields and farmers he has next to it.
Note that two-tile cities are two points for a knight when completed.
Third Edition Scoring
Count the number of farmers in that field. The player with the most
pawns inside the field will score for that field. If many players tie
for majority of pawns, they all receive the full points.
A field will score 3 points for each completed city adjacent to the
field. A city is adjacent to the field when part of the city walls are
used to define the boundary of the field (i.e., next to the field).
Each field is scored for itself. Therefore, a city may score more than
once for a player.
Note that two-tile cities are four points for a knight when completed.