I don't fully understand the question - are you saying you want the server to generate a 404 error instead of 403? Or are you asking how to configure a custom page for 403 error code? I don't think you are asking the latter since you already know how to configure one for 404.
Thanks Jaikiran for your reply. I have resolved this by adding another error-page configuration for 403 in web.xml.
<error-page> <error-code id="error403">403</error-code> <location>/error404.jsp</location> </error-page>