in_category conditional statement

Hello, I’m trying to show different sidebars on my single template depending on what category the post is in. I’m using a conditional statement and am wondering if this is achievable as I can’t get it working…

<?php if ( in_category( '18' )) {
include (TEMPLATEPATH . '/specialist-expertise-sidebar.php'); }
elseif ( in_category( '4' )) {
include (TEMPLATEPATH . '/industry-expertise-sidebar.php'); }
else {
if (in_category('4') && in_category('18')) {
include (TEMPLATEPATH . '/combined-expertise-sidebar.php'); }
}
?>

So to explain:
If the post is in category 18 then show specialist sidebar.
Or if the post is in category 4 then show the industry sidebar.
Or else if the post is in BOTH category 18 and 4 show the combined sidebar.

The else statement is where I’m struggling to target both of the categories…it just goes blank instead and none of it works.

Also I’m using in_category but unsure of the correct use here, whether I should be using is_category instead?

Can anyone advise and help to get this working? Thank you ??