4.—(1) The Commissioners may exercise the function under section 13 of arranging for evidence in England and Wales or Northern Ireland to be obtained under section 15 where—
(a)a request for assistance has been made in connection with criminal proceedings or a criminal investigation mentioned in section 14(1)(a), and
(b)those proceedings or that investigation relate wholly or mainly to a relevant offence.
(2) For the purpose of making a decision about a request for assistance the Commissioners must exercise the functions in section 14.