There’s some debate out there and it depends who you ask. To me 4PL is when a company takes over completely the responsibility for managing their client’s logistics. The 4PL typically communicates through all channels (air, sea, warehousing) and often may select the partners the client is using for their transportation.
This website does a pretty good job of describing it. If you Google ‘what is a 4PL’, you’ll hear a variety of answers. My take is that pretty much everything about managing logistics is outsourced to a single point of contact/partner.