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>