1. ACR 6.3 is
not compatible with CS4.
2. The latest ACR compatible with CS4 is ACR 5.7
3. ACR 5.7 is available for manual download from
here.
4. Read the installation instructions very carefully.
5. I've never heard of ACR updates being compressed in a WinRAR format - Adobe always uses ZIP. However, a great tool for decompressing any archive format (and compressing in seven types) is 7Zip -
free download here. It's a very handy little program.