RAD is very treatable in a four year old. There are a few attachment therapists that travel to do intensive attachment therapy, but you'd still need a primary attachment therapist for follow up.
My youngest child has been with me about 18 months. He spent the first 31/2 years in 3 different Romanian orphanges and the next 15 months with a single parent who had two jobs who kept leaving him in "respite" for 4 or 5 weeks at a time while looking for a new family for him. He was heavily medicated on Depakote
(500mg daily), clonodine(which he od'ed on in respite), and buspar. I held him every day and kept him with me all the time he was awake. He's about to finish kindergarten now, is on no meds, has no learning problems and shows very few signs of any remaining attachment issues.
FAS is a lifetime issue. Even though RAD mimics some symptoms,
with time and treatment, most of these kids will succeed.
Untreated RAD can be very serious, but most kids with RAD do respond to treetment and they do heal. I know there have been horror stories(I have one of my own), but these are the rare cases. Your son was diagnosed early, that's a blessing. Find a support group if you can. We found it very helpful.
Visit
www.radzebra.org . It's run by a friend of mine and she was running a group on line.
Make sure you give yourself enough breaks and find time to have fun with your healthier children. It helps fill you up so you don't get all drained out.