4 levels behind. When the half-fiend character has 10,000 XP, he's a 5th-level character, just like anyone else with 10,000 XP. (He just has one class level and four ECL levels.) So, he needs 5,000 more XP to get to 6th level-at 15,000 XP, he'll be a 6th-level character, although with just two...