You were taught wrong, but it was close, and the difference may or may not change the actual installation requirements.
A 500 MCM copper conductor is good for 380 amps. If the load is 380 amps or less, then you can use a 500 MCM. If the load is 381 amps or higher, you cannot use a 500 MCM. If you have not calculated the load, and you plan to use a 400 amp overcurrent device to protect a 400 amp panel, then a 500 MCM will not be sufficient. The point is that you must first determine the load, that you next select a conductor that has at least that much ampacity, and finally you select the overcurrent setpoint to protect the conductor.
That said, if you are allowed to use a 500 MCM, then you can protect it with a 400 amp overcurrent device (the next higher standard size). You can also protect a pair of parallel 500 MCM sets with an 800 amp breaker (again, the next higher standard size). But with three or more sets of parallel conductors, the overcurrent device will be rated over 800 amps. At that point, you can no longer use the next higher standard size, you must have an overcurrent trip setting that is no higher than the conductor ampacity. Reference 240.4(C).
Welcome to the forum.
Charles E. Beck, P.E., Seattle
Comments based on 2008 NEC unless otherwise noted.