I have this setup. The 345mhz IQ panel 4 and the 345mhz firefighter. Bought the firefighter from the Surety store (the round 345mhz version, non-encrypted).
I can confirm that it registers alarms for both the Smoke and CO tones on the Nest Protect and works 100% as expected, even with the verbal announcements between tones. Tested a couple times with canned smoke to be sure.
Very happy with this setup. Nest is still the only smoke detector I know that gives you a heads up before an alarm. Has saved me on multiple occasions from going into alarm mode over some toast haha.