What is fourth-party logistics (4PL) ?

It is when a firm which has two or more 3PL's working its logistics and then hires another company to coordinate and control the activities of the 3PL's,

Arrangement in which a firm contracts out (outsources) its logistical operations to two or more specialist firms (the third party logistics) and hires another specialist firm (the fourth party) to coordinate the activities of the third parties.