「船積み月末締切り125日後支払い(B/L Date 月から4か月後の5日)」に相当する英語表現は、

The accounts shall be closed at the end of each shipping month and due 125 days after that (on the 5th day of the 4th month after the month including the B/L date).

ではなく、

The accounts should be settled within 125 days following the end of each shipping month (by the 5th day of the 4th month following the month in which the Bill of Lading date falls).

が適切です。