(4)Any assessment of the extent of the disablement resulting from the relevant employment injury may also be reviewed by a medical board if it is satisfied that since the making of the assessment there has been a substantial and unforeseen aggravation of the results of the relevant injury:Provided that an assessment shall not be reviewed under this sub-section unless the medical board is of the opinion, having regard to the period taken into account by the assessment and the probable duration of the aggravation aforesaid, that substantial injustice will be done by not reviewing it.
