4G Masts do not connect via FTTC PCP cabinets, they are fed via a leased line directly back to the nearest exchange/POP. It might be located right next to your green FTTC cabinet because that's where the nearest BT ducting is, or where the nearest T-Node happened to be making life easier for all involved ;-)
(That, or the cabinet could have come AFTER the 4G mast now BT Openreach had to pull fibre for the mast).