C4, in this sense, is the written short name for Channel 4, a British television channel. It is how the channel writes its own name, and how British people write it in listings and online: “It’s on C4 tonight.” You do not usually spell it out in a long sentence; it works like a label. This is UK English. Students in Taiwan may never see it unless they watch British TV. Channel 4 is known in Britain for being a bit more alternative than the older BBC channels — though actually, that is a reputation, not a rule for every programme.

英國第四頻道（Channel 4）。台灣看到 C4，多半會想到炸藥；這個義項不是那個。