The frequency difference meter would be good for CW signals. I don’t know how it’ll perform with 4FSK. You might try our modulation analyzer in zero span for that. I think I’ve seen around 1 Hz typical error on readings there with time bases locked.
For a CW, using the frequency difference meter, a typical accuracy would be 0.01 ppb plus time base error, so in your case, +/- 1.01 ppb. At 440 MHz, +/- 0.4444 Hz.
Adding modulation complicates things a lot.
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