O is the centre of the circle.

AOB = EOF = COD = and AB = 4 units (Given)

Here, AOB = EOF = COD

Therefore, AB = EF = CD = 4 units [ If the angles subtended by two chords of a circle at the centre are equal then the chords are alos equal]

Thus, EF + CD = 4 units + 4 units = 8 units.