- Voltage level
The power management IC (PMIC, see section 3) is a MXL7704 with an input voltage range of 4.0 V to 5.5 V and an absolute maximum rating of 6 V that must not be exceeded.
Considering that the downstream USB ports are directly connected to the 5 V power rail the 5 V supply should also comply with USB electrical specifications: 4.45 V to 5.25 V (USB 3.0)
(TBC): Furthermore the "Power good" pins of the PMIC used on the Pi 3B+ (and thus likely on the Pi 4 too) was used to trigger the low voltage warning at 4.63±0.07 V (Raspberry Pi Power Limitations).
The reasonable voltage range to power the Pi therefore is: 4.7 V to 5.25 V
- Current requirement
Raspberry Pi 4 spec recommends a 3 A supply (15 W), with a minimum current of 2.5 A if downstream USB peripherals consume less than 500 mA in total.
The Foundations blog shows this picture of the power management section on the PCB. The PMIC is a MXL7704 - the same IC is used on the Pi 3B+.