Hi - I need to buy a return ticket from Futian Checkpoint to Shenzhen North.

Yes, you are right. The subway fare is CNY4 and the travel time is about 20 mins. Buying the tickets on the automatic ticketing machine, you need prepare change like CNY5, CNY10. If you do not have any change, in the passenger's service center, you can exchange some.

Thanks Sally for such a quick and helpful reply!