M.M.
My son is also 4 with high functioning autism, he is VERY clingy for the last couple of years now. HOwever, when I drop him to school he has gotten better, I prepare him while on the way to the school, I say remember, mommie is just gone for a little bit and I'll be right back, I even give the visual of holding my had apart approx 10 inches and then put my hands together when I say then I am already back, this seemed to help my son and he even repeats after me, you'll be back in a little bit right? It seems to ease his fear of separation from me. Good luck, hope this helps.