Have you spoken to someone at your local school? I'd probably start there; see what options they have available and what tests they will accept.

We are choosing to homeschool, despite living in a great school district, because we just don't think that it makes sense to expect a school to accommodate a child like ours (also 4 and reading at at least a fourth grade level). We may decide to dual enroll, but once we hooked up with our local homeschooling community, that seemed less crucial.