(iv) during the fourth year, to the extent of six and a quarter per cent. in value of the said sum: Provided that, if the Central Government so directs in any case, the securities specified in clause (b) shall be regarded as approved securities other than Government securities for a longer period than four years, but not exceeding six years in all, and the manner in which and the extent to which the securities shall be so regarded shall be as specified in the direction.