That could actually create a problem because having the step sticks short out when the USB is connected but the main PSU is powered down is a known issue.
It can personally testify that it is not an urban myth or mere opinion because it happened to my printer and I posted manufacturer's driver chip spec sheets with their warnings to this effect.
A snap noise, a nasty smell and bye bye the weakest step stick the current path could find.
If you want to power the board through the USB port, you should remove the step sticks first.
It is because the stepper chip circuitry gets back-biased through the motor supply pin and the chip may or may not tolerate that gracefully (for very long anyway).
It is far more common with any variety of TMC21xx step sticks but it also has been known to happen with 4988-based step sticks.
It also may damage traces or components around the USB connector due to overcurrent.
Photos of this very sort of damage have also been posted.
In other words, don't try this at home......
If it has worked for you, you got lucky.
There is a simple solution though.
I have posted photos of a 1N4002 diode that bridges the +5 and +motor pins of the unused second extruder socket (cathode toward the +motor pin).
It will solve the problem permanently and causes no ill effects.
It prevents the step chips from being back-biased.
Curiously, the smaller control board on the Ender-4 has just such a prevention diode built in (I noticed it right away).
A dirt cheap board has one and ours doesn't.
I completely agree with you on this sort of thing in general being commonplace on "first in hand" printer kits.
Part of the learning curve I guess.
@cosmobott, don't be discouraged.
Just work through it logically.