You're unable to accelerate in 4th gear right? Feels as if there's no power in 4th gear... it accelerates well in 3rd gear till wht ever speed and then when you stop accelerating, it shifts into 4th gear and then the comes down again to 90-100.
If yes then its common in the vitz/platz on cng. I dont know the exact reason but mine used to stay around 90-95 in 4th gear with 5 people + cng but you can slightly improve it by doing the things below... Now my vitz in 4th gear accelerates perfectly between 90-115 max. (note that if you start running it on petrol and tune it properly then it won't have any issues like this)
what you need to do is, make sue that
-Your air filter is not choked.
-your throttle body is serviced and CAT is removed and plugs are in good condition with proper gap.
-All Tires properly inflated at 28 or 30psi
And lastly but most important, your cng setting is tuned for performance and not economy. Make sure that your air intake is also blocked about 34-40%. Its not recommended by the experts but in this particular model of vitz, it helps in a way that the car changes gears at higher rpms and idling becomes much smoother. (If you have a proper venturi then its better not to do anything)
For tuning, you can view this thread...
just make sure that you're not limited the car's ability to reach full rpm by limiting the in flow of cng. hope it helps..