Land Rover Repair: discovery 2000 es auto seems to take along time getting into 4th gear, shuttle valve, dip stick

Question
Hi i have a discovery 2000 x reg ES td5.
When i start the car it will go into 1,2,3,but not 4th gear on the auto transmission until about 3-4 miles of driving then it is ok.
I had the transmission fluid and filter changed at 122000 miles and now the car has done about 130000, is it just a case of the fluid needs to warm up before it all works ok or maybe the fluid is low...? there is no dip stick to check the fluid and if it was low or leaking it would not work in 3rd then 2nd gear but the first 3 gears seem to be ok and 4th will take a little longer to get into.
Do i have a major problem ??
If you could give me any advice it would be very much appreciated     thanks pete

Answer
The delayed change up into 4th gear would lead me to believe the valve body has a stuck shuttle valve or a faulty solenoid. I would put an additive in the transmission and drive it for week or two. If it is still not working properly, you will have to have it investigated further.