What is a grade 4 ankle sprain ?

Severe sprain: with tearing of multiple ankle ligaments. possible tendon rupture, possible avulsion fracture, with associated severe pain, swelling and limitation of motion. Often associated with large hematoma formation.

Ankle sprain: In general, ligament injuries are graded 1, 2 or 3 to differentiate a stretched ligament from a partially torn ligament all the way to a complete rupture of a ligament or multiple ligaments. Grade 4 is not a routinely used term.

NO SUCH THING: I had to do a search, to make sure there isn't something new I missed. Ankle sprains are graded I-III.