Hello Connie,
It's interesting that the 1.2 has a gearshift indicator - the 1.4 does not, though my other car does.
The gearshift indicator is only a general guide - the car doesn't know the driving conditions that are coming up next. Often it will suggest a shift up into 5th, but there is no point doing this unless you expect to maintain the same speed, or a slightly decreasing speed, for a reasonable distance. If you're about to speed up or slow down, the indicator does not know this - and it's better to stay in 4th.
The best technique for economy is to accelerate quite briskly through the gears (not redlining, but keeping between 2000 and 4000 RPM, because that's when the engine is most efficient) to get the speed you need, then change into 5th just to maintain the speed or coast along with accelerator pedal completely released. The more time driving with accelerator released, the less fuel used. Also, the gearshift indicator will not appear.
Driving slowly with constant slight acceleration prevents the chance of any coasting, and is not as economical - and, the gearchange indicator will often appear.
So really, the exact gear you are in matters less than the indicator suggests. It's more important to get off the pedal early, and avoiding the brake pedal is the best way to save fuel.
-Alex