You're calculating the raspi 4 power consumption based off of recommended USB power supply current rating. The actual expected load power consumption without peripherals is 1/5 to 1/3 of that (3-5 W rather than 15 W).

https://www.raspberrypi.org/documentation/hardware/raspberry...

https://www.pidramble.com/wiki/benchmarks/power-consumption

https://raspi.tv/2019/how-much-power-does-the-pi4b-use-power...