4. Rights exercisable only in certain circumstances or temporarily incapable of exercise
(1) Rights which are exercisable only in certain circumstances shall be taken into account only—
(a) when the circumstances have arisen, and for so long as they continue to obtain, or
(b) when the circumstances are within the control of the person having the rights.
(2) Rights which are normally exercisable but are temporarily incapable of exercise shall continue to be taken into account.