hi
I was right

, see my post number 2 above. BERU still lists glow plugs with a 4 mm thread, but I believe this is a catalog error, as they look more like engine glow plugs (the longer ones) in the picture. However, the text clearly describes them as coolant glow plugs (BERU part number 0100 226 199, which costs €20.40 + 16%). Source: tec-cat.
You could also take one of your spark plugs out and look at the number stamped on it to see if it matches the numbers I posted.
I don't need to explain where you can buy Bosch or BERU products.
Good luck.
gerrit