Will taking antivirals like tamivir help prevent pneumonia even if not started until day 4 or 5 of the flu?

Not likely: Tamivir is active against specific viral pathogens and not bacterial. If you are trying to prevent bacterial pneumonia don't smoke, maximally treat asthma if you have it, maintain good diet, exercise regularly, maintain a healthy weight and be certain to receive a vaccine for pneumococcal pneumonia at the appropriate age (appropriate according to insurance companies).