custom field on category page

Hi,

This is the php code I use to get my custom field on category page:

<p><?php the_field(‘author’, ‘category_4’); ?></p>

But this displays the value of category_ID_4 on all my categories.

How should I do to display the value of each corresponding category_ID ?

Many thanks in advance!

http://docstories.net/category/stories/ (cat_ID 2)
http://docstories.net/category/matteo-gariglio/ (cat_ID 4)

http://wordpress.org/extend/plugins/advanced-custom-fields/