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Use the last segment of your index finger to bar strings 2, 3, 4. Above string 1 your index finger is slightly raised (bend your finger at the joint) so it mutes the string rather than fretting it.
It is also possible to have your finger raised enough above string 1 to have it ring out open, but that is more difficult and tends to result in the string buzzing, so better to just mute it when first attempting this barre.
