**Core Concept**
The management of breast cancer involves a multidisciplinary approach, including surgery, radiation, and systemic therapy. In this scenario, the patient has undergone surgical removal of the tumor with a margin of 4 mm, which is considered adequate for most breast cancers. However, the presence of high-grade necrosis suggests aggressive tumor behavior, and additional treatment may be necessary to prevent recurrence.

**Why the Correct Answer is Right**
The correct answer is radiation therapy, as it is often recommended for patients with high-grade necrosis in the surgical specimen. Radiation helps to eliminate any residual cancer cells that may be present in the surrounding tissue, reducing the risk of local recurrence. In this case, the patient's tumor size and grade, combined with the presence of necrosis, make her a good candidate for radiation therapy.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the patient has already undergone surgical removal of the tumor, which is the primary treatment for breast cancer. Additional surgery is not typically necessary unless there is evidence of residual disease or a margin of less than 1 mm.
**Option B:** This option is incorrect because the patient's tumor is already being treated with surgery and radiation. Systemic therapy, such as chemotherapy or hormone therapy, may be beneficial in certain cases, but it is not the next step in this patient's management.
**Option C:** This option is incorrect because the patient's tumor is not described as being in a location that would require a mastectomy. A mastectomy may be necessary in some cases of breast cancer, but it is not the next step in this patient's management.

**Clinical Pearl / High-Yield Fact**
When managing breast cancer, it's essential to consider the tumor's biology, including its grade, size, and presence of necrosis. Radiation therapy can be an effective way to eliminate residual cancer cells and reduce the risk of local recurrence, especially in patients with high-grade tumors.

**Correct Answer: C. Radiation therapy.**