Please be more specific. What exactly isn't working? What operating system? If you're on Mac, naturally the plug-ins may not show up. AFAIK BCC 4 is PowerPC only whereas CS4 is intel only. Therefore neither can work together and upgrading at least to BCC 5 would be required. If it's something else, then more info is required.
I received this answer from email@example.com
"Premiere Pro CS4 requires BCC 6.
An upgrade would need to be purchased in order to move from BCC 4 to BCC 6.
A BCC 6 AE upgrade is a cost or $295 and can be found on our web store at
the following link:
http://www.borisfx.com/store/cart.php?target=category&category_id=28 or by
contacting our sales department at 888-772-6747.
Let me know if you have any other questions."
Don't know if this is true or they just want me to buy the update?
They sure won't say No to selling an upgrade, but that aside, I think they are sincere. There must be technical reasons, especially in relation to their OpenGL accelerated plug-ins vs. Premiere's and AE's use of hardware acceleration. I just cvannot give you any solid information on that, I'm afraid. I have so many plug-ins from other vendors, I couldn't tell you if I even had reason to use BCC even if it was free. Just so many identical tools and functionality overlap... Maybe someone else can chime in.
BCC added support for CS4 with the 5.0.1 release.
The BCC upgrade cost is the same regardless of wether you move to 5 or 6 so your best bet, if you want BCC to work with CS4, is to upgrade to BCC6.