I must be getting better at this stuff. I was thinking, what have I come across that would do what I think WE Premultiply was supposed to do. I used Shift Channels and set the controls to: From Luminace - To Alpha and I was able to make the black solid with the Lens Flare effect have and Alpha with transparency. So I guess if Shift Channels was not around in 2004, it makes Premultiply obsolete.
However, when I compare the results to using lens flare effect alone on a black solid and the Screen Blending Mode, there is a difference in the falloff of the rings of light and color. The blending mode has more colors and more rings of light. Turning up the Flare Brightness with the Shift Channels does not restore the lost color. Any alternate ways of dropping out the black background an making it a transparent alpha.????
You're correct that the old AE 6.5 plugins are just too old for CS4 compatibility. Note that even now we're all getting around the fact that CS4 plugins don't work in CS5!
There are a number of ways to unmultiply/premultiply layers. I usually precompose the layer in question, then duplicate the precomp-ed layer and assign it as Luminance Track Matte to the first.
The most common third party Lens Flare Plugins, Red Giant's Knoll Lens Flare and Video Copilot's Optical Flares, both have unmult capabilities built into them, which is why many standalone unmult plugins haven't been updated over the past few years.
But Red Giant Software still offer a free version of UnMult. It's labelled as AE 7 compatible - you could try it and see if it works in 6.5.
Thanks for that link. I was able to use the Knolls UnMult in CS4.