It seems perfectly logical and consistent to me.  {4} is a repeat count, as is *.  You get the same error if you do 'a?*', and the same bypass if you do '(a?)*' (though I haven't tested if that does anything useful :).  You don't need the ?:, as far as I can tell, you just need to have the * modifying a group, making the group the "preceding regular expression".