You do need a licence if you watch any tv live as it's broadcast, or BBC iPlayer.
But you'd need a licence if you only ever watch Channel 4 and never watched any BBC content. The licence is not tied to BBC content - you don't need a licence to use the website or listen to BBC sounds or use BBC radio.
The licence is tied to watching live tv as it's broadcast or BBC iPlayer.
There's a TV licence, but that's payable if you watch any live tv as it's broadcast.