The problem is that javax.faces.ViewState is not rendered when you render another form.
But if you take a look inside richfaces.js you will see following in richfaces.ajax = function(source, event, options):
parameters.render = "@component";
so if you use a4j:commandButton than context.render="@component" - JSF just skips that...
Possible workaround will be to render a part of the form, not whole one,
or modify jsf.js to check if context.render==="@component" use list of all form ids in your document separated by space instead.
P.S. Richfaces 4.2.1.CR1, Mojarra 2.1.7