Even as far back as the Rpi 2, they violated the USB spec by allowing more than 500mA to downstream devices. IIRC, you had to alter the config.txt file to alter the current limiter on the USB ports. Later, when the USB spec allowed for higher currents, they followed suit. IIRC, it’s 900mA on a USB3 type A–unless you implement USB-BC then I think it’s 1.9A or such.
USB type C connectors are completely different animals and would need a wikipedia article sized reply to explain.
So, you should be good on the Pi4 is you use one of the USB3 ports, but you’ll want to check the power specs of your drive. If it’s >900mA, you may need one of those cheater cables that plugs into two USB ports. That’s likely the case if it’s a mechanical HD, maybe not if it’s an SSD.
Okay, theory is great, but here’s an actual test: Thomas Kaiser said that he hit a current limit of 1.1A when powering deivces from the Rpi4. He states that’s the sum total allowed for all USB ports combined. You might be needing that external powered hub after all.
