Example, i drive in city, where speed varies from 20-70km/h. If engage D4, it will shift to 4th gear when reach 50km/h, after shift up, the RPM is around 2k only (actually could be lower, only around 1800rpm, but since it is torque converter, slippage cause engine reading is higher), so when continue accelerate to 70km/h, i need to press throttle quite deep due to ratio not correct.

Please bear in mind, this will lead to poor FC even RPM is low. Because @ 50km/h @ 4th gear, i need to press quite deep throttle to accelerate, in this case, the RPM will shoot up higher which is 500 rpm higher due to torque converter slippage, and the fuel is wasted in this way.

So i engage in L3 in this case to lock in 3rd gear only, and only turn to D4 when speed reach >70km/h @ around 3.5k rpm, i notice only need light foot on pedal and acceleration is alot better. Sometimes i even use L2 to lock 2nd gear until RPM >3k only shift to L3, then >3k only to D4. For downshift, i will shift from D4 to L3 once it <60km/h, if in D, it will not shift down to 3rd gear. The result is a superb FC, around 14.5km/L in 100% city drive. Don forget this car already aged 11 years. Even my gf surprise her car can get such low FC in city drive.

I'm not sure about Saga 4AT, but hope to share my experience in short driving period of 4AT. Not sure whether this will harm the transmission or not. My understanding this will not because what gear you select is not direct link to gearbox(unlike manual), it just tell ECU to shift down or up. Hope this can improve FC on Saga 4AT, and drive 4AT quite fun actually.