I had to password recover a Cisco ISR 4451, and kept having issues getting into the ROMMON prompt. Every guide mentioned sending a BREAK character during startup, but I could not get that to work. I was using the mini-USB port in the front, and as far as I knew did not have password recovery disabled. It turns out there is a problem with the mini-USB port and the Mac driver, I switched to using a traditional serial cable with a DB-9 connector/RJ45 serial port and suddenly I could get into ROMMON. I wanted to post incase anyone else runs into this.
Below is the startup process, at the end there you should be able to send a BREAK character.
Initializing Hardware ... System integrity status: 00000610 Rom image verified correctly System Bootstrap, Version 15.3(3r)S1, RELEASE SOFTWARE Copyright (c) 1994-2013 by cisco Systems, Inc. Current image running: Boot ROM0 Last reset cause: PowerOn Cisco ISR4451-X/K9 platform with 4194304 Kbytes of main memory Warning: filesystem is not clean File size is 0x1d482044 Located isr4400-universalk9.03.16.04b.S.155-3.S4b-ext.SPA.bin <SEND BREAK HERE>