It's not super narrow but it's narrower than Terminal 2 if you are asking for the entire width of the north wing. The walkway on 4F is definitely narrower cause it opens space for looking down to 3F, plus there are now two people movers. Utilizing satellite from google maps, if we don't include the protruding area towards the terminal/car road that the toilets take space, it's about ~25 meters in terms of width. ~28 meters at areas where there are orange/red protruding toilets.

Terminal 2 is about ~34 meters from side to side for Section D North Wing, not including the protruding area near the terminal connectors. ~23 meters from side to side for section C South Wing, ~28 meters if we include the protruding toilets.

Terminal 1 is about ~7 meters for the walk way, some areas have protruding shops which is another ~7 meters, ~28 meters if we include the area where there is a boarding gate (but that's only 1 floored space).

Sterile area is all connected and it seems like they have reserved space for future expansion should Terminal 2 turns into 4 story as well (last photo where trentusjuventus posted a dead end on the 4F to reach elevator), so this will stay connected and change if Terminal 2 goes under construction in the future. When the main terminal opens in Terminal 3, passengers will clear immigration on the 4F at the main terminal before heading down to 1F for bags and customs. It's not as simple as the current set up where it's all on second floor. The future setup passengers will arrive at 3F, take the escalator to 4F (right now it's to 2F in Terminal 2), walk to terminal to clear immigration, down to 1F for bags and customs.

You do get some extra space on 2F in the wings of Terminal 3, but we don't know how much of the ~25 meters can be used for passenger facilities.