Solution:
(i) {1, 2, 3, 4}
{$x: x$ is a natural number and 4 ≤ x ≤ 6} $=$ {4, 5, 6}
As a result, we obtain
{1, 2, 3, 4} ∩ {4, 5, 6} $=$ {4}
As a result, this pair of sets is not disjoint.
(ii) {a, e, i, o, u} ∩ {c, d, e, f} $=$ {e}
As a result, {a, e, i, o, u} and {c, d, e, f} are not disjoint.