If you can’t get your copy of Parallels 4 working under Snow Leopard because it quits on launch you could try dropping into the Terminal and then doing
cd /usr/local/lib
sudo ln -s /Library/Parallels/Parallels Service.app/Contents/Frameworks/ParallelsVirtualizationSDK.framework/Versions/3.0/Libraries/libprl_sdk.dylib .
Worked for me!