Subtracting previous images in sequences

I have a sequence of images of migrating and dividing cells. When the cells divide, the nucleus changes from black to white. I would like to count the number of cell divisions. To easily locate cell divisions, I thought it would be useful to subtract from every image, the image recorded 4 images earlier. In this way, constantly white areas should disappear while it should be easy to locate areas where the color changes quickly from black to white. Does anyone know if such a procedure can be easily performed and whether an automated count of cells divisions can subsequently be performed for the whole sequence?