firstly you have a 1.6 GLi and it came with a tachometer, you posted a picture of a car which had no tachometer. It would also be very helpful if you told us that if the overdrive is not switched off, when its switched off the transmission will only shift 2 times - meaning it will run in 3rd gear (also known as direct drive)
the 4th gear is overdrive and is achieved under normal driving conditions when the correct speed is achieved in relation to the control pressure in the transmission. The overdrive switch works backwards - meaning it shuts off supply to the 4th clutch and also disengages the sprag when you press the button. No feed from O/D off = all 4 gears are achieved.
- So back to the same question - what is the rpm you see when you are at 100 km/h - and how many shifts did you count to that speed. - 4 gears = 3 shifts.