There are (at least?) two types of headrest: the 'soft' ones (covered in material) and the solid 'hard' ones. I think diameter of the legs are different.
Our 1990 GTL built in Slovenia has 'hard' headrests - 'soft' ones sourced in the UK wouldn't fit.
My advice is to check the distance between the legs and also the leg diameter to avoid buying ones that don't fit.
See https://www.renault4.co.uk/forum/threads/hard-soft-headrests-wanted.698/#post-11214