Acute Appendicitis: The presentation of right lower quadrant abdominal pain, fever, pallor, and tachycardia, especially with the pain radiating to the right upper quadrant, is highly suggestive of acute appendicitis. The progressive nature of the pain over 4 days also aligns with this diagnosis.

Mesenteric Lymphadenitis: This condition can mimic appendicitis and presents with similar symptoms, including abdominal pain and fever. It's often related to viral infections.
Ovarian Cyst or Torsion: Although the pain is more typically pelvic, ovarian pathology can sometimes present with right lower quadrant pain, especially if the cyst is large or the ovary is torsed.
Diverticulitis: While less common in this age group, diverticulitis can cause severe abdominal pain, fever, and changes in bowel habits, though the latter is not present in this case.

Ectopic Pregnancy: Although the patient does not mention any history suggestive of pregnancy, ectopic pregnancy can present with abdominal pain, pallor, and tachycardia due to internal bleeding. It's crucial to rule out pregnancy in females of childbearing age.
Intestinal Obstruction: This condition can cause severe abdominal pain, vomiting, and changes in bowel habits. While not all symptoms are present, the severity of pain and systemic symptoms (fever, tachycardia) warrant consideration.
Perforated Visculitis or Ulcer: Conditions like perforated ulcers or vasculitis can lead to severe abdominal pain, fever, and systemic illness. These are less common but critical to diagnose early.

Right Lower Lobe Pneumonia: Referred pain from a lower lobe pneumonia can sometimes present as abdominal pain. However, this would typically be accompanied by respiratory symptoms.
Hepatic or Renal Pathology: Conditions affecting the liver (e.g., hepatitis, liver abscess) or kidney (e.g., pyelonephritis, renal abscess) can cause right upper quadrant or flank pain, respectively, but could be considered if the pain radiates or if other symptoms suggest these organs are involved.
Inflammatory Bowel Disease (IBD) Flare: While IBD (Crohn's disease or ulcerative colitis) can cause chronic abdominal pain and changes in bowel habits, a flare could potentially present with acute worsening of symptoms, including fever and systemic illness.