My period is late by 4 days. I have all the signs of pregnancy. My temperature is 37 c. But, all my four tests came negative. Could I be pregnant or not?

Maybe. Wait until you are a week late, the check a first morning urine. If you are in a hurry, get a serum( blood) test.
Stop Testing. Four tests negative are a very good sign that one is not pregnant. But if your symptoms persist it is time to get the blood test at your family doctor or gynecologist's office.
Keep on trying! I presume you and your husband have been thoroughly examined for any pathology of your reproductive organs. Best wishes! http://www.Ehow.Com/list_6858298_medicinal-herbs-conception.Html.