PCo doesn't have a license as such, you obtain usage rights by having licenses for other products such as MII / ME etc. You need to double check with your SAP Account Exec but the last usage rights I saw stated that if you had an ECC license you could use PCo but not with S4HANA.
Depending on the use case I would typically recommend using MII in between as it makes life easier if you have to make multiple calls to S4H and also make it easier to call your RFCs/BAPIs without having to expose them as webservices. It can also act as a buffer for when S4HANA is down for a maintenance window.
Hope this helps