An isosceles trapezium is always :

1. a parallelogram

2. a square

3. a rectangle

4. a cyclic quadrilateral

Given,

ABCD is an isosceles trapezium with AB ∥ CD and AD = BC.

∠A + ∠D = 180° [Interior angles on the same side of a transversal are supplementary]

In an isosceles trapezium,

∠A = ∠B and ∠C = ∠D [Base angles are equal]

So,

∠A + ∠C = 180°

Since a pair of opposite angles of quadrilateral ABCD is supplementary,

ABCD is a cyclic quadrilateral.

Hence, an isosceles trapezium is always a cyclic quadrilateral.

Hence, option 4 is the correct option.