If you compare the dates each volume was published with dates of documented events in their marriage, I think it is safe to assume that Maria likely did not have any part in writing the 4th volume. Their marriage was on the rocks pretty early in the 1880s. It appears that as CTR became more fanatical with his views and self-aggrandizing, the more conflict there was in their marriage. I do think volume one was either a joint venture between the two, or completely penned by Maria.
Studies in the Scriptures:
Volume 1: The Divine Plan of the Ages (1886)
Volume 2: The Time is at Hand (1889)
Volume 3: "Thy Kingdom Come" (1891)
Volume 4: The Battle of Armageddon (1897)
Volume 5: The At-One-Ment Between God and Man (1899)
Volume 6: The New Creation (1904)
The Marriage of Charles and Maria Russell:
1979 - Married
1895 - Maria claims this was the first year that Charles first suggested separation
1896 - Charles and Maria disagree about the editorship of the paper. Russell wrote letter to Maria saying their marriage was a mistake. He said, "For the past three years you have been cruelly forcing upon me the evidence that we both erred when we married."
1897 - Separated in November
1897-1909 - Drama, drama, drama until separation/divorce was final