$1 = source
$2 = target
$3 = k
$4 = directed

SELECT
        *
    FROM
        pgr_ksp(
         'SELECT id, source, target, cost, reverse_cost FROM osm_2po_4pgr',$1, $2, $3,
directed:=$4)

SELECT
    *
FROM
    pgr_ksp(
     'SELECT id, source, target, cost, reverse_cost FROM osm_2po_4pgr as r,
             (SELECT ST_Expand(ST_Extent(geom_way),4) as box FROM osm_2po_4pgr as l1
            WHERE l1.source = ' || $1 || ' OR l1.target = ' || $2 || ') as box
            WHERE r.geom_way && box.box',$1, $2, $3, directed:=$4
    )

SELECT
    *
FROM
    pgr_ksp(
     'SELECT id, source, target, cost, reverse_cost FROM osm_2po_4pgr as r,
             (SELECT st_buffer(st_envelope(st_collect(geom_way)), 4) as box FROM osm_2po_4pgr as l1
            WHERE l1.source = ' || $1 || ' OR l1.target = ' || $2 || ') as box
            WHERE r.geom_way && box.box',$1, $2, $3, directed:=$4
    )