Lassaigne's sodium-fusion test detects nitrogen, halogens, sulphur and phosphorus in organic compounds by fusing them with sodium metal.

(1) Forms NaCN — used to detect nitrogen via Prussian blue test. ✓ part of Lassaigne.

(2) Forms Na${}_2$S — used to detect sulphur via lead acetate/sodium nitroprusside. ✓ part of Lassaigne.

(3) Forms NaX — used to detect halogens via silver halide test. ✓ part of Lassaigne.

(4) 2CuO + C $\to$ 2Cu + CO${}_2$ is the reaction used to detect carbon (combustion test, not Lassaigne).

$\therefore$ Option (4) does not belong to Lassaigne's test.