Update post meta function

I’ve got 2 quick related problems that I’d love to get some help on.

First, I have a PHP function that is supposed to track clicks of a hyperlink and store the values in a post’s custom field. The tracking code is:

function qwe4_tracker() {
	global $post;
			// get each post by ID
			$postID = $post->ID;
			// get the post's custom fields
			$custom = get_post_custom($postID);
			// find the view count field
			$views = intval($custom['qwe4'][0]);
			// increment the count
			if($views > 0) {
				update_post_meta($postID, 'qwe4', ($views + 1));
			} else {
				add_post_meta($postID, 'qwe4', 1, true);
			}

}

So far it works, but the problem is that the function is updating every post’s custom field together instead of individually. How do I get it to update only the post with the link that is being clicked?

Second:
How do I properly execute the function only when the hyperlink is clicked?

Currently I have:
<a href="<?php qwe4_tracker(); ?>">link</a>
which runs the script on every page load instead of on the actual click.

Thanks!