Here are the only instructions I've come across for the wagons on Marco's list, and only then with the pipe wagon, 4105. I have bought off eBay boxed examples of 4100, 4105, 4107 and 4111, but have only found these instructions in the 4105. (I have two boxed ones of these.)
As you can see, with the instructions there also comes a peg, rather like a miniature clothes peg. I say "a peg", but with one of the 4105s there were two pegs. The instructions by themselves aren't over clear, but when new the peg is fitted to one of the wagon couplings as shown in 1. To couple the wagon to another wagon, you have to remove the peg. When you have coupled the wagons together, you replace the peg and it prevents the two wagons coming uncoupled over uneven track. To uncouple the wagons, you have to remove the peg first.