I'm on my 4th day with covid, fever was gone yesterday, today pass with no fever and now at night the fever is back, how long does the fever last? also is it better acetaminophen or ibuprofen?

It is hard to predict when the fever will be gone. As long as the infection is still active then fever is still present. In severe cases, fever may still persist after 10 days. So far, we've known when fever has gone for more than 24 hours, the infection is most likely subsided and you are no longer an infectious host. Both noted drugs are effective but in general acetaminophen is probably safer.