"I had the same problem with just one motor/esc, the other 3 boot up just fine. The 4th motor boots up with the others with their usual set of beeps, but then it gets stuck beeping forever and will not power up when I arm the other motors they…"
"That does seem odd. I'm assuming your magnetometer has the components down, pins forward?
Is it possible that you have magnetic interference? Either from outside the aircraft, or more likely, from within, say, the ESCs, maybe?