I have this route:

category_set_representative_image:
  url:     /categoria-settare-immaggine-rappresentativa
  param:   { module: category_new, action: setRepresentativeImage }
  class:   sfDoctrineRoute
  options: { model: Category, type: object }

and a link associated to this:

url_for('category_set_representative_image', array('sf_subject' =>
$category))

When I click the link I get:

    sfError404Exception: Action
"categoriasettareimmagginerappresentativa/index" does not exist.

Any idea?

sf 1.4

http://www.symfony-project.org/api/1_4/UrlHelper#method_url_for

This basically means that you should use the url and not the route
name as a first parameter, in your case:

url_for('category_new/setRepresentativeImage', array('sf_subject' => $category))

Or, if you want to make it a little faster, you can use directly the
route name as in:

url_for('@category_set_representative_image', array('sf_subject' => $category))

Hope it helps,

Kind regards.