how to ignore internal 404 response and only activate a irule on a full page 404 response ( such as css, image etc. )

Yes, there is... Just identify the requests that represent full page as opposed to those that represent page component :)

Maybe "full page" is only these:

/ /index.html /index.php /products /products/ /products/index.htm

You could go either I think, but you'd more or less have to define all page objects or all non-page objects, or at least object extensions. Here's an example that defines non-page objects from a data group.

when HTTP_REQUEST {
    if { [class match [string tolower [HTTP::uri]] ends_with objectlist] } {
        set do_not_err 1
    }
}
when HTTP_RESPONSE {
    if { [info exists do_not_err] } {
        unset do_not_err
        return
    } elseif { ( [HTTP::status] starts_with "40" ) or ( [HTTP::status] starts_with  "50" ) } {
        HTTP::respond 200 content "Broken"
    }
}

where objectlist is a string-based data group that contains the lowercased extensions of known object types that you might encounter on a page. Example.

jpg
gif
png
js
css    

If you page 'breaks' when you get a redirect to a maintenance page for an includes-type object, then I suspect that it will 'break' when you get a 404. However to answer your actual question this should work;-

when RULE_INIT {
     List of well known 'page' extensions
    set static::l_ext [list "" "do" "htm" "html" "jsp" "asp" "aspx" "wml" "xhtml" "json" "cfm"]
}
when HTTP_REQUEST {
     Save variable for possible later use in HTTP_RESPONSE
    set request [HTTP::uri]
}
when HTTP_RESPONSE {
    if {[HTTP::status] == 404 && [lsearch $static::l_ext [string tolower [getfield [URI::basename $request] "." 2]]] != -1 } {
    HTTP::respond 302 noserver Location "http://mycomapny.com/sorry_404.htm"
    return
}