4. The number of the instalments specified in any statement under paragraph 3—N.I.
(a)if the effective date of the notice under Article 29(1) is after 30th April, shall in the year in which that effective date falls be not less than the number of complete months between that date and the end of that year (including, if that date is the first day of a month, that month) reduced, if that number of months is three, by one or, if that number of months is four or more, by two;
(b)in any other case shall be not less than ten;
and the date specified in any such statement for the first instalment thereunder shall, not be earlier than ten days after the date when the statement is sent, and the interval between any two of the instalment dates shall not be less than one month.