I was trying to do the same thing, and i could really find a solution for NN4.x
I highlighted a cell or DIV, by changing its bqckgrouund color, then i needed to change it back to transparent.
You could try setting the cells background color to whatever is the backgroudn color of the layer beneath your cell.
If you have an image underneath your cell, then its more tricky, you have to crop a part of the image, and put the cropped part as background image.
If you use JAvascript for the background color change, then you can separate netscape 4.x browsers, and use a different effect, for those who are still using this old browser.
Hope that helps,
Fuli42