Feel free to do what you want. If the barrel has already been cut down then there is not much you could do anyway to restore it to a normal No4 without replacing the barrel.

Anyway, Jungle Carbine (No5) and No4 stocks and handguards are interchangeable from what I know, except maybe that a No5 stock may bite into the reciever where a No4 stock does not, since the No5 reciever is milled in certain places to save weight. Flash hider I am not sure where you could get one, or even how they mount on the barrel. You could probably find a No5 butt-stock around to make it look even more like a No5, or just keep the No4 since it will probably fit the socket a bit tighter, assuming it is the original one.

Just keep in mind that if you had a No4 MkIII or No4 MkI/2 (I believe that is correct) you would not be able to swap a No5 fore-end onto it because of how the trigger mount is changed.

I'm pretty hazy on No5's, and even No5's, but no one else has helped yet, so if I were you I would wait for someone a little more knowledgeable to have a say.