The Raspberry Pi 4 now only uses one, which is why it is being detected as an audio adapter.
Whether your USB-C charger works with the Pi 4 has to do with whether it uses an "e-marked" cable.
The Raspberry Pi 4 has been lauded for its improved specs while keeping the low price tag of the original. More specifically, it doesn't work with so-called e-marked cables, such as those used on Apple Macbooks and several other laptops.
The issue was first reported by Liliputing and has now been confirmed by Raspberry Pi co-founder Eben Upton in a statement to ZDNet sister site TechRepublic. Incidentally, the new iteration does not have a dedicated power supply port and has to completely depend on USB Type C port. But they would have if the Raspberry Pi Foundation had simply followed the USB-C specification to the letter. Since the Pi 4 USB-C port is wired incorrectly, these smart cables will detect the Pi 4 as an "Audio Adaptor Accessory" and refuse to charge them. Upton added that he expects the issue to be fixed in a future board revision but for now, users will need to apply a workaround [use a cable that isn't e-marked] to circumvent the problem. "It's surprising this didn't show up in our (quite extensive) field testing program".
The Raspberry Pi Foundation recently launched the fourth iteration of its immensely popular single-board computer, the Raspberry Pi 4 Model B. Soon after, several units purchased by eager buyers failed to power up through the USB Type C port.
Leung has long back criticized Raspberry Pi makers for seemingly failing to have done sufficient testing. Blogger Tyler Ward first discovered that there was "incorrect detection circuitry" in the Pi 4's connector. The system doesn't power up. Had they simply mirrored the USB-C spec, this wouldn't have happened. It features a powerful 1.5 GHz quad-core 64-bit ARM Cortex-A72 CPU, up to 4GB of RAM, support for up to 4K resolutions, Bluetooth 5.0, Gigabit Ethernet, 2x USB 2 and 2x USB 3 ports, 2x micro-HDMI ports, and a USB-C power supply.
An e-marked cable uses a chip to make sure that the cable can handle the power or data transmission requirements of the intended use.