The Disk Cache is not used for RAM previews. It can be used for Standard Previews (hitting the space bar instead of RAM previewing).
If you'd like to use the Standard Previews as opposed to the RAM previews, you may not get realtime results.
How big is the cache you've allocated?
Generating a RAM preview should result in AE starting to cache frames at the beginning of your work area, and proceeding to cache frames towards the end of your work area for however much RAM is available with which to preview. If you had previewed content at some point at the beginning of your timeline, and then adjusted your work area to start a few seconds in, then the previewed content at the beginning (the green line) will be incrementally removed to cache new frames in your new work area.
How much RAM does your machine/setup have? Maybe take a gander at the CS4 help files on this topic.
All that being said, I rarely use, or see effeciency or consitency in using the disk cache. Only on a few projects have I found it to be useful/effective.
That's all subjective, I suppose. When I was on a workstation with Nucleo Pro (made by Gridiron), there was some more effiecnt background rendering/cachiing of the timeline occuring that was useful, but it came with a processor hit.
or not at least the way it used to work for me under CS3. when I was on CS3, i reuced the RAM setting for ram previews down to the lowest setting. then what would happen is that all my previews would immediately cache to disk and wouldshow up as a nice blue line telling me that this was happening. I was very happy with this setting as it allowed me to do long previews.
Actually that behavior is incorrect in CS3 and to add to the oddness, it cannot be reproduced on any system. More down to the point: It's an arbitrary bug. RAM previews should only be commited to disk, if another preview is initiated, effectively overwriting the other frames stored in the RAM. Therefore CS4 is doing what is expected, when not even initiating previews. Also consider, that the disk cache is only used, when AE >thinks< using the stored frames would be faster than re-rendering. Since CS4 uses more OpenGL features, this will also not always be the case. That being so, you will have to adapt your workflow to deal with the new behavior. Nucleo could be an option, but it is not supported for CS4, so you are pretty much left alone with the matter.