My kids are probably not doing anything that resembles what their friends in PS are doing, but we just call them the grade that matches their age. That's what people really want to know anyway. So my kids are in 2nd and 5th grade, regardless of what work they are doing.
You could go ahead and call him a 4th grader, and define the work he is doing as 4th grade work, and say he is right where he needs to be. He is, and he's doing fine now, which is great.