Since 當期 is 一期, then 二期 is the last period, i.e., the period before the present period. Consequently, 三期 is the period before last period. Since we have got 三期 defined, then we may say 四期 is the period before 三期, and 五期 is the period before 四期.

imho:
In addition to the present period, the reports of the 4 periods before are required for the 資產負債表, they are the last period and 3 more periods prior to the last period. The nomenclature
of the periods are as follows: The periods are named in reverse time order as periods 1-5 (from the present to the past).

From then on, each period may be referred to by its number.