That sounds awesome. Unfortunately it looks like CS4 no longer has that option…
I found a hack to address this. Open the swf and scale it up as before (to 5242×1080 in this case). The swf will only display the left 4000 pixels, the remaining 1242 pixels on the right get cropped. Duplicate the swf layer and precompose it. In the precomp, slide the swf to the left so that the right side of the animation is now in the middle of the comp. Back in the original composition, line up the precomp with the original swf layer so it looks like there’s only one layer. Basically use a seperate peice of the animation to patch the hole that popped up because of the continuous rasterization problem.
A friend told me this problem has to do with SWFs having a size limit of 4000×4000.