This sounds it *could* be the common problem of an extra space after a submit button in a form within certain browsers. I do not have NS4 on this machine but try removing the submit button temporarily to see if it has any effect.
You have no submit button defined so ignore the above
In order to get around such a problem, consider using a TD with no graphics and structure the table so that no matter what discepiances there are with the way forms are rendered, your page doesnt break. Including a form element in a 100% sliced header is a bad idea IMO.
Alternatively, here is some great reading on Forms in HTML about, how, why and solutions to common problems.