Ford Territory won't go into third and fourth gear

As with any automatic transmission problem, the first thing to do is check the level of transmission fluid in the gearbox itself. These shouldn’t need top-ups, but if a leak occurs, they can lose enough fluid to play havoc with the gear selection and drive-transmission processes. Low fluid can cause a loss of drive in any gear, but could also contribute to a valve-body problem where the lack of fluid prevents the correct gear selection process. Low fluid can also mean the clutch-packs and bands can not exert enough pressure to get the car moving.

It's worth noting, though, that this particular transmission has a finite lifespan and a loss of third and fourth gears could be the first sign that the unit is worn out and requires either replacement or reconditioning. A transmission specialist should be your first port of all.