A specialist registrar is a doctor undergoing specialist training in a hospital.
Grades 4-6 means in years 4-6 of training - in other words, near the end of the training period. Other grades here means junior specialist registrars - years 1-3 of training.
Terms such as "chef de clinique" do not correspond exactly. I think it would be best to stick to specifying the years of specialist training.
Note also in the new UK system, it's "specialty registrar" and no longer "specialist registrar"
http://www.wessexdeanery.nhs.uk/specialty_schools/arcps/rita...