On a cantilever sliding gate, the counterweight section is the part of the gate panel that sits past the runout-side post (the post the cantilever rail rides over). It exists to balance the gate's weight: when the gate is closed, the counterweight tail and the gate panel both sit on opposite sides of the roller fulcrum.
Rule of thumb: the counterweight section should be 30 to 40% of the opening width. A 4 m clear opening gets a gate that's 4 m of panel plus another 1.4 to 1.6 m of counterweight tail, so the gate panel is closer to 5.5 m total.
The counterweight section can carry battens or slats as a continuation of the gate face (it ends up sitting along the fence line when the gate is closed) or it can be left as a bare cantilever rail with no infill, which is cheaper but uglier.