@abdulrehmanrauf ! 40km/h on 4th gear is EASY. Its all about throttle control. BUT, as you pointed out, changing gear on low speeds will be jerky, and if rider is not experienced, he might stall the engine in doing so.
So yeah, you CAN change the gear on low speeds BUT its better to change the gear at its intended speed, and THEN bring down the bike at 40km/h.
Personally, i DON'T ride the CG125 in 4th Gear at that speeds. 3rd Gear suits more. CG125 has this strange beemari ... where it feels like, it has lost its torque and power (very obvious in hot weather conditions), if you continue to ride it in "high-gear/low-speed" condition. Can't really explain that problem in words, but experienced riders might know this, as CG125 is unpredictable in giving you Power at a Specific Gear, say 3rd.