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The SM4 and Mini 2 share the same microphone and recordings of very similar quality. For a typical use case of recording animal vocalizations to identify species, the differences between the two recorders are insignificant.
For noise-level measurement or other uses where the greatest signal-to-noise ratio is a priority, the SM4 has several advantages. This knowledge base article comparing the SM4 and Mini 1/2 covers these advantages in greater detail.