Sorry.. been away for a while.... re-building a house y'see.
Relax: it's not a big problem.
Firstly, the reason it drops out of 4WD when he de-clutches is because there is no more load on the dog-linkage... letting the selector lever succumb to gravity.
Have a look at this link:
I've taken photos from the Foton Parts catalogue that came with my FT254
Follow the 4WD selector toggle to the linkage on the transmission under the footplate. Get your mate to pull it up and down: you'll see two detent holes (in part 1) into which a ball (part 2) should seat to maintain the selected position.
The Lever (part 4.2) has a threaded hole running though it, and further into the hole, a brass lining (4.1). My guess is that the bolt (part 7) that should keep the spring (3) and ball in place has dropped out.
You'll need to find a new ball and spring that slip neatly into the hole, and pop in a new nut and bolt to keep them in place.
Don't forget to grease it...
Have fun.
(When this happened to mine, I used a bungee to hold the selector up in 4WD)
All the best from Sweden,
/Nick