According to [1] a 1 meter USB cable can have a resistance of 286 milliohms, for a voltage drop of 0.8v over a meter (taking the resistance of four contacts and two lengths of cable into account).
So if you have a '3A 5V' power supply without a captive cable, you can still get 4.1v at the useful end of the USB cable.
The official RPi chargers include a captive cable, presumably to stop users substituting such standard USB cables.
[1] https://www.westfloridacomponents.com/blog/compensate-cable-...
Re:Cooling off your Raspberry Pi 4
by imtringued on Friday November 15, 2019 @04:37AM
(#21543562)
The raspberry pi sells power supplies not chargers
A lot of the USB chargers most people see are way lower than the rating marked on them. That is why you should buy a special charger for your stuff, not the generic cheap stuff at the supermarket.
the whole point of a 3A rated 5V power supply is it will not drop more than ~5% under its rated load. What you describe is being a victim of a fraud.