Upset stomach and diarrhea for 4 days could it be coronavirus?

Usually not diarrhea: It is not normal to have diarrhea, so one can look for the usual causes of temporary diarrhea, with temporary meaning it will go away in a day or a few days. Causes may include stomach viruses (but usually not COVID-19), contaminated food, medication effects, any food that just didn't agree with the stomach or intestines, etc... When symptoms are bad or prolonged, a doctor can evaluate.

Diarrhea: If you do not have high risk exposure, stomach upset and diarrhea are perhaps more likely due to common gastroenteritis or other reasons than coronavirus infection. Approximately 4% of COVID-19 infected patients had diarrhea.