a4j:region is not a NamingContainer . It is designed to be transparent. Use f:subview.
P.S. I hope you understand that a4j:region does not limit what is submitted, but only what it is processed. I.e. having one huge form might increase the traffic from client to server.
I understand, thanks. If I have a huge page and specific areas to be submitted and decoded I must use several forms with one or more a4j:region if need.