Very short 4th gear.....?

The 4.0 goes throughought 1,2 & 3rd gear normally in normal mode (auto) but when accelerating on a motorway or dual carriageway it holds 4th gear for all of two seconds then goes to fifth at low revs (1500k) my box changes around 2krpm at normal acceleration but it doesnt seem to like 4th and goes to fifth almost straight away. Any ideas people, Even under half throttle, it holds it for a very short while.. Anybody elses do this or similair?

Does it have 5 gears?Thought the ZF was four gears and the "fifth" gear is the torque converter locking up.

If it's a P38 it only has 4 gears, torque converter lock up occurs around 50 mph unless you really boot it.

thanks guys.