On a cantilever sliding gate, the counterweight section is the part of the gate that sits past the post the rail rolls over. It is there to balance the gate. When the gate is shut, the tail and the main gate sit on opposite sides of the rollers, like a see-saw, so the gate stays steady.
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.