Viral Gastroenteritis: The patient presents with watery diarrhea for 4 days without fever, which is a common presentation for viral gastroenteritis. The lack of fever and the absence of blood in the stool (not explicitly mentioned but implied by the focus on watery diarrhea) further support this diagnosis. The patient's lab results do not indicate a severe infection or inflammation that would suggest a bacterial cause.

Bacterial Gastroenteritis: Although less likely due to the absence of fever, bacterial causes such as Clostridioides difficile (especially if the patient has been exposed to antibiotics recently), Salmonella, or Campylobacter cannot be ruled out without further testing. The elevated white blood cell count (WBC) could suggest a bacterial infection.
Food Poisoning: This could be due to consuming contaminated food or drinks. The symptoms and lab results provided do not specifically point towards food poisoning, but it remains a possibility, especially if the patient has recently eaten outside or consumed potentially contaminated food.
Irritable Bowel Syndrome (IBS): Given the chronic nature of the patient's anemia (as indicated by low hemoglobin and hematocrit levels in previous tests), IBS could be a consideration, especially if the diarrhea is recurrent and there are other symptoms such as abdominal pain or bloating.

Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease or ulcerative colitis can present with chronic diarrhea and may lead to anemia due to chronic blood loss or malabsorption. The patient's history of low hemoglobin and hematocrit levels could be indicative of chronic disease.
Celiac Disease: An autoimmune reaction to gluten, leading to malabsorption and potentially causing chronic diarrhea and anemia. This diagnosis is crucial not to miss due to its implications for long-term management and the need for a strict gluten-free diet.
Giardiasis: An infection caused by the Giardia parasite, which can lead to chronic diarrhea and malabsorption. It's particularly important to consider in individuals with exposure to contaminated water or travel history.

Whipple's Disease: A rare, systemic bacterial infection caused by Tropheryma whipplei, which can lead to malabsorption, diarrhea, and weight loss. It's rare but important to consider due to its severity and the need for specific antibiotic treatment.
Microscopic Colitis: A condition characterized by chronic, watery diarrhea, often without blood, and can be associated with inflammatory changes in the colon that are only visible under a microscope. It's more common in older adults and can be a diagnosis of exclusion after other causes have been ruled out.