Hi there, was reading the documentation and it mentions:

The Breast Cancer Histopathological Image Classification (BreakHis) is composed of 9,109 microscopic images of breast tumor tissue collected from 82 patients using different magnifying factors (40X, 100X, 200X, and 400X). For this benchmark we only use the 40X samples which results in a subset of 1,995 images.

Seems like the BreakHis mentions the true magnification. So they multiple the eyepiece magnification (10X) with the objective magnification (40X).

See figure on their website:

As you can see, the 40X here is 4X.