MGA and his team had thought about writing BA-5 in 1907, this quotation seems to be the final word on the book-series. This was found by Hani Tahir and posted.
“””Is maslehat se khuda ne Quraan sharif ko 23 baras tak nazil kia ta k us muddat tak mouda nishanat bhi zahir ho jawen. So main yaqeen rakhta hoon k kitab baraheen e ahmadiya ki takhir par abhi 23 wan baras khatam nahi goga k is ka panchwan hissa mulk main shaiy ho jaiga aur khuda taala ne baraheen ahmadiya mein 23 baras ki taraf ishara bhi kia hai..”””
He means if Allah swt took 23 years to complete the revelation of Quraan then barheen ahmadiya will also be complete and published within 23 years and it part 5 will come in that period as this has been told to him by Allah swt.