Since January experts at Sotheby’s have been trying to privately sell a suite of four bronze sculptures of a woman’s back that Matisse created over a period of 23 years beginning in 1908. The works belong to the Burnett Foundation in Forth Worth, and at the time the sale was announced, neither Sotheby’s nor the foundation would say what the asking price was, although dealers approached by the auction house reported that they were hoping for around $200 million for all four.

Now, as the fall auction season approaches and the sculptures remain unsold, Sotheby’s is announcing that it will auction the four works separately — in chronological order — over the next year, starting with the earliest and most classical of them all, “Back I,’’ which will be included in its Impressionist and modern art auction in New York on Nov. 2. It is estimated to bring $20 million to $30 million.

The foundation decided to sell the sculptures in November after another cast of the fourth back in the series brought $48.8 million at Christie’s. It was then, officials for the foundation said, that they realized the value of the four works far exceeded anything that could justify owning them, since its mission is to support health, education, human services and arts initiatives mostly in the Fort Worth area.