IF specific entry for select element is selected…

So I am using some code in my functions.php file to alter the subject of the email form when sent. I can get it to send just what is selected from the subject dropdown just fine and I can get it to include what is put in the “Other” field. But I haven’t been able to find a solid way to only target the “Other” option from the dropdown/select element so that what is entered into the “Other” text field is only added to the subject ONLY IF the user selects “Other” from the select element. Any ideas?

Here’s what I have currently…

// Changes the Enfold form subject
add_filter('avf_form_subject','avia_change_mail_subject', 10, 3);
function avia_change_mail_subject($subject, $new_post, $form_params){
$from_name = urldecode($new_post['1_1']);
$old_subject = urldecode($new_post['3_1']);
$subject = $old_subject .' - '. $from_name;
$other = urldecode($new_post['4_1']);
	if ($other){
	return $subject . ' - ' . $other;
	}
	else {
	return $subject;
	}
}