Yes. NS4 will only show the background color on the part of a div that has content. IE and NS6 do not have this problem. So you need to use a workaround. Using layer-background-color along with clip:rect values is one way to get around it.
I believe another solution would be give the div a solid border of 0.1px. Such a narrow border would not show up on either browser but would cause NS4 to fill in the div with the background color even if there is no content in the div. However, I don’t find this second solution preferable because of another bug in NS4 that causes a small space (about 3px) between the border and any content you do have in the div. *sigh*
Of course you could also fill the div with non breaking spaces, but that would be very tedious.