(1) The taxpayer's taxable year under this article shall be the same as his taxable year for federal income tax purposes.
(2) If a taxpayer's taxable year is changed for federal income tax purposes, his taxable year for purposes of this article shall be similarly changed.
(3) The taxpayer's method of accounting under this article shall be the same as his method of accounting for federal income tax purposes.
(4) If a taxpayer's method of accounting is changed for federal income tax purposes, his method of accounting for purposes of this article shall be similarly changed.