Is your car on CNG? If yes, then you need to perfectly tune your vitz and block the air intake by 40%. I had same problem on the highway..the 4th gear didnt seem to work so the car was changing again and again back to 3rd. On high speeds there is a lot of air passing into your engine through air intake manifold and CNG requires very less air to burn as compared to petrol. Zolojazz, a member here, suggested me to block the air intake pipe and it did work wonders for me. Try it by blocking it with a fiber tape...you will feel the difference in the sound of the engine and in the throttle response.
If no CNG in your car..then just get your throttle body serviced.