The problem is they are adapting a book series to make the show. You ask why she can't be in season 4? Cause she ain't in the book that they are basing season 4 off of. You make a hash outta stuff when you delete or replace an important character in an adaptation. Like in the related Harry Bosch series starring Titus Welliver, they deleted Mickey Haller (bosch's half brother) and replaced him with a female character that never appeared in the books (Honey). Then they doubled down and basically rewrote the stories to expand her role. It messes everything up.