score:6
Your mother is allowed six months per visit, as you know. Leaving after six months for just a few days, returning for another four months will look very much like she's trying to live in the US. She risks being refused entry.
This is at the discretion of the border guard, who is required to presume immigrant intent. She may have been allowed entry under what appears to be similar circumstances before, but that's no indication of what will happen now.
As your mother makes more visits, and the visit length increases, it's more likely that she will be denied entry.