"Was a hazard?"
Yes. The GFCI would not work as expected. At best it might behave like a non-GFCI outlet. It could also cause a phase-to-phase short and trip the circuit breakers, as you noticed.
"Could my readings and diagnosis be wrong?"
Yes. Multitesters are cheap and easy to use. If you keep one probe on a ground wire at all times, it's nearly foolproof.
Edit: In the comments to this answer, it was noted that at least one of the circuits involved here has its neutral tied to multiple other circuits. This could cause a misleading indication from a "NCV tester" as there could be AC potential on the neutral wires even with the hot wires open.
"The master bathroom is also on breaker #4 and it has multiple GFCI outlets."
This is the key here. You have 2 circuits and multiple loads involved. While you might think this one receptacle is wired wrong, it's entirely possible that one of the other junction boxes is housing the home run for both circuits and that's where the connections are wrong. So I encourage you to check basically everything on those two circuits.
"How can I trace breaker #6?"
There are two practical ways to go about this. The cheap and time-consuming way is to turn off as many breakers as needed to figure out which circuit goes to every outlet. If you're willing to invest a little cash and learn how to use the tracing tools from the hardware store, you could narrow it down much more quickly.