How do I retrieve multiple values from a multidimensional array in wp_options?

The following should do it:

function get_hidden_cats() {
    $my_cats = get_option('ce4_category_fields');

    $my_hidden_cats = array();
    foreach( $my_cats as $cat_id => $cat_attrs ) {
        if( 'true' == $cat_attrs['my_cat_hide'] )
            $my_hidden_cats[] = $cat_id;
    }
    $my_hidden_cats = implode( ',', $my_hidden_cats );

    return $my_hidden_cats;
}