This is another area of ImageGallery that is going to get a good overhaul -- syndication of images to other pages.
Until those changes are implemented, I would just handle that with some display logic on the template.
<% if Pos lt 5 %>
<% end_if %>
Looks like you've extended the Album class. Make sure you isolate your changes in a new class so updates will not overwrite your changes. This would be a good opportunity for you to create your own GalleryItems function, too.
class MyImageGalleryAlbum extends ImageGalleryAlbum
static $db = array ('Featured' => 'Boolean');
return DataObject::get("ImageGalleryItem","AlbumID = $this->ID", $limit);
class MyImageGalleryPage extends ImageGalleryPage
protected $albumClass = "MyImageGalleryAlbum";